If attributes and the distribution of attribute points sketch out your character in broad strokes, skills are where you apply the finely painted detail to your heroic creation. You can check out all of the skills your character currently knows by hitting the K key or going to the main menu and selecting Skills. You can equip up to eight skills at once, creating a unique character "build" ready to handle most any situation. Most skills are linked to a particular attribute, and also carry a subtype such as Stance, Spell, Hex, or Shout.
Your hero can only equip skills or move those skills around on the Skill Bar when in a town. But when you're in town, your entire array of skills is available to choose from. The skill list is sort-able by a variety of options, including the attribute the skills are attached to, so you can be sure to select skills that fit the hero's key abilities. If your Necromancer has all available attribute points in Blood Magic and none in Curses, selecting a Curse skill is probably not the way to go.
You will hear the term "build" a great deal in relation to character skill selection in Guild Wars, especially in PvP play. Your character build refers to the specific assortment of skills and attributes that you have selected for a particular mission or quest. Many players tend to settle into a preferred build, while others like to adjust skills for every new enemy. The skills in a solid character build should work well together, and work well against the foes you expect to encounter. You should also take into account the other members of your group—it's a good idea to confer with the other players in your party so that there's a minimum of skill overlap. For example, if there are two Monks in the party, one may choose to set up as the healer for the group with skills linked to the Healing Prayers attribute, while the second acts as the party's protector, choosing a selection of skills tied to Protection Prayers.