The Skill Bar contains all your currently equipped Skills.
The Skill Bar has 8 slots. According to the official explanation this is because "most skills come in the form of a ring to be worn on the character's finger, which is why one can take only 8 skills into battle."
To execute one of the equipped Skills you can click on its icon or press its corresponding key. Skills are always numbered 1 to 8 on the keyboard, unless you change their shortcuts (in this case, the numbers disappear). Some skills will require you click on a specific target before execution, either an ally, a foe or a pet. HaX