World of Warcraft Skills
What are the categories of character skills in World of Warcraft?
Skills are grouped into several categories: professions, secondary skills, weapon skills, class skills, armor proficiencies, and languages. Your character's skills can be viewed on the skills tab of your character sheet. Professions are the major trade skills available to all classes, such as tailoring and mining. A character can only have two professions at any given time. Secondary skills, such as fishing and first aid, are also trade skills but have no restriction on how many can be learned. Weapon skills include all weapons that a character is able to use. Many weapon skills need to be learned, as your character only starts with a handful of skills he can learn. Class skills are determined by your character class (granted at character creation) and are an integral part of your character. Armor skills are known from character creation, or received from trainers or talents. Language skills are assigned at character creation and do not change.
How do you improve your skills and professions?
Basically, players improve their skills through usage. While class skills automatically increase when a character levels, weapon skills and trade skills only improve through usage. Thus, in order to improve your ability with swords, you must equip and fight with a sword. Professions and secondary skills operate similarly. In order to increase your skill level in a profession or secondary skill, you must use it.

What are skill caps and how do they raise?
Skill caps are the maximum level that you can currently raise a given skill. Skill caps are raised by leveling (weapon skills) and by receiving training (professions and secondary skills). For weapon skills, gaining a level of experience will raise the cap for each weapon skill by 5. For tradeskills, your profession rank - apprentice, journeyman, expert, artisan, and master - determines your maximum skill level. These ranks are gained by receiving training (from NPCs and/or books) and have minimum skill and level requirements.

Where can you learn professions and secondary skills, and how many can you learn of each?
Professions and secondary skills are learned from various trainers around the world. Some tradeskills will be rare and hard to find trainers for, while others will be much more common, with trainers located in multiple cities. Players can learn a maximum of two primary professions. In order to learn a new primary profession, a player must unlearn one of his or her existing primary professions first. Secondary skills are less versatile and robust than primary professions, but there is no limit to the number of secondary professions you can learn. For all tradeskills, you will be able to raise your maximum skill cap by receiving the next tier of expertise.

What kinds of items can players create using tradeskills?
Players can create a wide variety of items from resources gathered around the world - all using tradeskills. Players can make bags, bandages, weapons, armor, potions, and many other items with the tradeskills of Tailoring, Blacksmithing, Engineering, and others. The raw materials, including herbs, ore, and leather, are obtained using gathering tradeskills such as Herbalism, Mining, and Skinning. The finished products of these tradeskills can rank among the very best of their kind available.

Can you unlearn a skill?
You can unlearn tradeskills, but not any other type of skill. To unlearn a tradeskill, click the appropriate icon next to the skill name in your character's Skills tab. Since you can only learn two primary professions at any given time, the time unlearning a tradeskill will come into play is if you wish to drop a profession to learn another. You can relearn the initial profession if you wish, but your skill level in that profession will be reset to zero.

Do skills atrophy if they aren't used?
No. Once you learn a skill or improve a skill point, it's a permanent gain unless you decide to unlearn the skill (if possible).