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 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.

Bone is currently only available either through Premium subscriber shops (in the Sea Caves) or in the Paladin shop on Ratha. Cloth, like Bone armor, is currently only available either through Premium subscriber shops (in the Sea Caves) or in the Paladin shop on Ratha.
Leather is often used by Barbarians, Rangers, and Thieves. This is the most commonly used armor that least affects stealth.
Light Chain and Heavy Chain are used by those who want something that offers more overall protection than leather, but not the hindrance and weight of plate.
Light Plate and Heavy Plate are generally used by paladins because their weight and hindrance are often thought to be excessive for other guilds.
Shield is the ability to effectively protect one's self 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. At 50 ranks, you are able to load a bow or crossbow while wearing a shield on your arm.

Lore skills ...

Animal Lore is the ability to handle animals, including ranger companions, leeches, and horses. Fishing also teaches animal lore as well as foraging.
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 versus one's abilities. It takes perhaps 50 ranks to begin to see inaccurate appraisals of stats on creatures and armor/weapons.
Astrology is available only to Moon Mages and is the ability to study the stars for predictions and for the creation of enchantments.
Empathy is available only to Empaths and is the ability to heal others and to perceive health.
Mechanical Lore is the skill which allows characters to create items, such as origami, armors, weapons, and magical items. This also allows for changing the appearance of items and people, as with embroidery and shifting. You can learn this skill by foraging grass or vines and braiding them repeatedly.
Musical Theory is available only to Bards and aids in the performance of enchantes, instrument playing, and other Bardic abilities.
Percussions is the ability to play percussion instruments, such as castanets.
Scholarship is the ability to learn from classes, reading library books and compendiums, and studying spellbooks.
Strings is the ability to play stringed instruments, such as the lute.
Teaching is the ability to teach classes to others.
Trading is available only to Traders and encompasses many Trader-only abilities, such as running caravans, holding auctions, and selling gem pouches.
Vocals is the ability to sing and is best learned by humming. If you intend to sing, I suggest asking the permission of your audience first, however.
Winds is the ability to play wind instruments, such as the flute.

Magic skills ...

Primary Magic is the ability to cast spells effectively.
Harness is the ability to control mana.
Power Perception is the ability to see the amount of mana available. This ability is limited by your own ability as well as by the amount of mana actually present in a room. For more information, please click here. Power Perception differs slightly for Bards and Moon Mages.
Arcana is the ability to store magic in cambrinth and to manipulate all magical devices, such as 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.

Survival skills ...

Backstab is available only to Thieves and is the ability to ambush and stab an opponent when hidden.
Climbing is the ability to climb over obstacles or into some tricky area. You can also learn climbing by finding a wall or tree (or other appropriate area) and typing climb practice <object>.
Disarm Traps is the ability to disarm trapped boxes without injury. Please click here for more details.
Escaping is the ability to escape webs, guards, and other such things. One way to learn Escaping is to travel back and forth between Crossing; one room along the way often traps you in mud and forces you to dig yourself free.
Evasion is the ability to dodge attacks.
First Aid is the ability to successfully tend wounds and remove leeches. Improving your skill here allows you to tend more difficult wounds as well as reduces roundtimes.
Foraging is the ability to find objects, such as branches and herbs, in the wild. This skill can also be learned by Fishing.
Hiding is the ability to slip into hiding without being noticed. It's closely related to Stalking, which is listed below.
Lockpicking is the ability to pick open locked boxes. Please click here for more detail.
Perception is the ability to see things, creatures, and people which might otherwise be hidden or invisible.
Scouting is available only to Rangers and is the ability to track people and creatures. This can also be used to run trails. The verb hunt is a great tool for Rangers in particular as it trains Scouting, Perception, and Stalking in that order. (Non-Rangers will only learn Perception and Stalking.)
Skinning is the ability to skin and scrape hides, pelts, and skins. All guilds may now arrange a creature at least twice before skinning, which increases the difficulty.
Stalking is the ability to approach or follow a person or creature unseen.
Stealing is the ability to steal objects or coins from a person.
Swimming is the ability to move across bodies of water, both great and small.

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

Parry is the ability to avoid attacks by parrying with a weapon. Parry sticks and edged weapons work best.
Multi Opponent is the ability to handle multiple foes without suffering a penalty.

Light Edged is the ability to use small bladed weapons, such as daggers or dirks.
Medium Edged is the ability to use medium bladed weapons, such as scimitars and sabres.
Heavy Edged is the ability to use heavy bladed weapons, such as broadswords and longswords.
Two-handed Edged is the ability to use bladed weapons requiring both hands, such as claymores.

Light Blunt is the ability to use small bludgeoning weapons, such as cudgels.
Medium Blunt is the ability to use medium bludgeoning weapons, such as maces.
Heavy Blunt is the ability to use large bludgeoning weapons, such as flails.
Two-handed Blunt is the ability to use bludgeoning weapons requiring two hands, such as war mattocks.

Short Bow is the ability to use weapons such as horseman's bows.
Long Bow is the ability to use long bow type weapons, like forester's bows and hunter's bows.
Composite Bow is the ability to use reflex type bows.
Light Crossbow is the ability to use crossbows that require the shortest time to load.
Heavy Crossbow is the ability to use larger crossbows, such as siege arbalests.

The following weapons are not recommended for beginners.
Brawling is the ability fight unarmed by use of maneuvers, such as punching, choking, and biting.
Light Thrown is the ability to use small throwing weapons, such as throwing daggers.
Heavy Thrown is the ability to use large throwing weapons, such as spears.
Short Staff is the ability to use some clubbing type weapons, such as lead pipes.
Quarter Staff is the ability to use long, heavy staves.
Slings is the ability to use weapons which propel rocks.
Staff Sling is the ability to use weapons that are similar to slings, but which are mounted atop staves.
Halberds is the ability to use large pole-arm type weapons, which typically do slicing damage.
Pikes is the ability to use large pole-arm type weapons, which typically do puncture damage.

This section continues onto other considerations here.