A good thief is capable of hiding in plain sight
Normally a skill used by the dexterous, thieves are the common adopters of the hiding skill. Keep in mind that as dexterity is a need for hiding, cumbersome armor will prove to be unusable.
To activate hiding, use the skills ability gem. If in combat, you will require distance or a break of line of sight to hide from your foe.
Commonly macroed, this skill needs to be repeatedly used with a delay for the skill use timer. While you can start with or train this skill at a NPC, the ease and speed of training this skill usually finds people skipping these options.
Those who wish to hide in the shadows will find that at 80 hiding skill you are able to begin training stealth, the ability to move while hidden.