This is intended as a brief summary of each skill and is by no means meant to include all possible uses of the skills. Skill levels are generally referred to as ranks. For example, 50 ranks in Teaching allows one to teach 3 pupils.

Armor skills grant the ability to use a specific class of armor and lower one's regular hindrance and stealth hindrance while doing so. Increased skill allows one to move more easily and to increase the protection received.

Light Armor covers cloth, leather, and bone armor. It's the popular choice for armor tertiaries and stealthy characters, due to its generally low maneuvering and stealth hindrance.
Chain Armor covers light and heavy chain, offering more overall protection than light armor without the weight and hindrance of plate.
Plate Armor is the strongest armor in the game and is often used by Paladins. Its incredible hindrance makes it a less popular choice for those who want to use stealth.
Brigandine covers armor formed of sewn plates, including what is commonly called banded mail.
Shield Usage is the ability to effectively block attacks with a shield. You are able to wear a shield on your left arm by typing adjust shield. Paladins may arm-wear large shields because they are armor primary. Guilds with armor as a secondary skillset may arm-wear medium shields. Those with armor as a tertiary skillset can only arm-wear small shields.
Defending is your overall ability to defend against attacks, particularly when facing multiple opponents at once -- it's checked against an aggressor's Tactics skill. Your STANCE settings determine how your defensive ability is split between Evasion, Parry, and Shield.
Conviction is a Paladin-only skill that powers soul-related abilities and the Smite system.

Lore skills cover crafting, teaching, and a number of guild-specific abilities.

Alchemy is a crafting skill used to create potions, poisons, medicines, oils, naphtha, and other products from foraged herbs and other materials.
Appraisal is the ability to appraise objects, in order to learn things about them, such as weight and worth. Appraising creatures allows one to learn their difficulty compared to one's own abilities. It takes perhaps 50 ranks to begin to see (inaccurate) appraisals of stats on creatures, armor, and weapons.
Bardic Lore is available only to Bards and is used with their unique music-based abilities, such as screams and enchantes.
Empathy is available only to Empaths and is the ability to heal others and to perceive health.
Enchanting is a crafting skill used to create magical devices and add enchantments to other items.
Engineering is a crafting skill used to create complex mechanical items; lockpick carving falls under this skill.
Forging is a crafting skill used to create items from metal.
Mechanical Lore is an obsolete skill, replaced piece by piece with the release of Alchemy, Enchanting, Engineering, Forging, and Outfitting. It can no longer be trained; any ranks in it can be converted into the new crafting skills with the convert command.
Outfitting is a crafting skill used to create worn items, such as clothing, from cloth, leather, and yarn.
Performance is the ability to perform artistically, including playing instruments and singing.
Scholarship is the ability to learn from classes, reading library books and compendiums, and studying spellbooks. It's also used to more effectively teach others, and determines how much information you can obtain from the recall command.
Tactics is the ability to spot an opponent's weaknesses and exploit them. It's used to apply multiple-opponent penalties to a target, for non-violent combat maneuvers, and for the combo system.
Trading is available only to Traders and encompasses many Trader-only abilities, such as running caravans, holding auctions, and selling gem pouches.

Magic skills govern spellcasting and the use of magical devices.

Primary Magic is a guild's core magic skill, providing a bonus to spells of the caster's own magic type.
Attunement is the ability to perceive and tap into the mana streams that fuel spells. It replaces the older Harness Ability and Power Perception skills.
Arcana is the ability to manipulate items imbued with magical energy, such as cambrinth, runestones, magical fans, and wands. Use of constellation jewelry and invisibility rings also fall under this skill.
Targeted Magic is the ability to aim offensive spells.
Augmentation is the primary skill used when casting spells that buff yourself or others.
Debilitation is the primary skill used when casting spells that inflict negative status effects without causing direct damage.
Warding is the primary skill used when casting barrier and shielding spells.
Utility covers spells and abilities that don't fit neatly into the other magic skills.
Sorcery is the ability to cast spells using a mana type you're not naturally attuned to. Training it is not illegal on its own, but many spells cast this way still are.
Astrology is available only to Moon Mages and is the ability to study the stars for predictions and for the creation of enchantments.
Summoning is available only to Warrior Mages and governs familiar summoning, Aethereal Pathways, and the creation of elemental weapons and domains.
Theurgy is available only to Clerics and is the ability to channel devotion, gained through rituals, into communes with the Immortals.

Survival skills cover a wide range of self-sufficiency and stealth abilities.

Athletics is the ability to move through bodies of water and climb obstacles. It replaces the older Climbing and Swimming skills.
Backstab is available only to Thieves and is the ability to ambush and stab an opponent when hidden.
Evasion is the ability to dodge attacks and is one of the three primary defensive skills, along with Parry Ability and Shield Usage.
First Aid is the ability to successfully bandage bleeding wounds and remove foreign objects, such as arrows or parasites, that have lodged in the body. Improving your skill here allows you to tend more serious wounds with less roundtime.
Instinct is available only to Rangers and is used to track creatures and players, and to read trailmarkers on the Ranger Trail System. It replaces the older Scouting skill.
Locksmithing is the ability to pick locked boxes and disarm their traps without injury, as well as carve and fix lockpicks. It replaces the older Lockpicking and Disarm Traps skills. Please click here for more details.
Outdoorsmanship is the ability to find items, such as branches and herbs, in the wild, and to interact with animals, including ranger companions, leeches, and horses. It replaces the older Foraging and Animal Lore skills. Fishing also teaches this skill.
Perception is the ability to see things, creatures, and people which might otherwise be hidden or invisible. It's the main defense against stealthy attackers, pickpocketing, and backstabbing.
Skinning is the ability to harvest hides, pelts, and other materials from slain creatures. All guilds may now arrange a creature at least twice before skinning, which increases the difficulty.
Stealth is the ability to slip into hiding without being noticed, remain hidden, and move while hidden. It replaces the older Hiding and Stalking skills, and is contested against Perception.
Thanatology is available only to Necromancers and embodies a mixture of the study of death and dying, gross anatomy, and occult or demonic rites.
Thievery is the ability to steal coins and objects from a person, as well as shoplift and pilfer from containers. It replaces the older Stealing skill.

In this section, defensive and mastery weapon skills are placed first, followed by the types of weapons most commonly used, and lastly weapons not recommended for beginning players.

Parry Ability is the ability to fend off an incoming melee or pole-range attack with the weapon in your right hand. Parrying also teaches some of the skill of the weapon you're parrying with.
Melee Mastery grants a bonus when using melee weapons, especially helpful during backtraining. It's best trained by training multiple weapons of that type.
Missile Mastery grants a bonus when using ranged weapons, especially helpful during backtraining. It's best trained by training multiple weapons of that type.
Offhand Weapon is the ability to wield and attack with a (non-brawling) weapon in your off hand, commonly called dual wielding. Two-handed weapons cannot be used in the offhand.
Expertise is available only to Barbarians and represents their ultimate combat mastery, powering Barbarian Combat Maneuvers.

Small Edged is the ability to use small and medium bladed weapons, such as daggers, dirks, and scimitars. It combines the older Light Edged and Medium Edged skills.
Large Edged is the ability to use larger one-handed bladed weapons, such as broadswords and longswords. It replaces the older Heavy Edged skill.
Twohanded Edged is the ability to use bladed weapons requiring both hands, such as claymores.

Small Blunt is the ability to use small and medium bludgeoning weapons, such as cudgels and maces. It combines the older Light Blunt and Medium Blunt skills.
Large Blunt is the ability to use larger bludgeoning weapons, such as flails and morning stars. It replaces the older Heavy Blunt skill.
Twohanded Blunt is the ability to use bludgeoning weapons requiring both hands, such as war mattocks.

Bows is the ability to use short bows, long bows, and composite bows.
Crossbows is the ability to use light and heavy crossbows.

The following weapons are not recommended for beginners.
Brawling is the ability to fight unarmed, by use of maneuvers such as punching, choking, and biting, as well as improvised weapons.
Light Thrown is the ability to use small throwing weapons, such as throwing daggers, as well as thrown items like naphtha and dirt.
Heavy Thrown is the ability to use large throwing weapons, such as spears and throwing hammers.
Staves is the ability to use quarter staves and short staves. It combines the older Short Staff and Quarter Staff skills.
Slings is the ability to use slings and staff slings. It combines the older Slings and Staff Sling skills.
Polearms is the ability to use large pole-arm weapons, such as pikes and halberds. It combines the older Pikes and Halberds skills.

This section continues onto other considerations here.