The Thief is the 'shady' indirect combat job (class) in the world of Vana d`iel (Note: An indirect combat job is a job whose primary purpose is to support other jobs in combat). Thieves possess many underhanded skills necessary for survival - instead of fighting their enemies from the front, they prefer to stab them in the back, use their quickness to evade attacks and steal items and gil (money) from them.
Parties like Thieves for a variety of reasons. Thieves can sense enemies from far away. Enemies tend to drop more money and items (about 10% up) when mid-level Thief is present. At 15th level, Thieves gain the ability to sneak attack, doing massive amounts of damage to the back of a monster who is not looking at (noticing) you. These abilities (in addition to their ability to sprint away from danger) make the Thief an ideal candidate for scouting / finding monsters for the party to kill.
Thieves do not make good front line fighters (lower hit points / defense) - they prefer to have a Warrior or Monk around to take the hits while they use their special Thief abilities to damage / harass the monster. At mid to high levels, Thieves do much better in a party than they do solo.
Many low level Thieves use daggers (or other quick weapons) because of the ease of which these weapons quickly "skill up" (less damage but much quicker attacks). This can be useful, as Thief is one of the more difficult jobs to level up in solo play (similar to a white mage).
Thieves have strong defensive skills: they get both Shield (allowing them to block attacks with their shield) and Parry (allowing them to parry attacks with their weapon).
Many players enjoy having Thief for their support job (level 15) because of their unique skills (steal, sneak attack, complete dodge etc).
The Thief job can be a challenge, but with the right mindset, they are definitely a fun job to play. If you want something unique, the Thief job might be for you.
Thief combat skills: hand to hand, dagger, sword, club, archery, marksmanship, throwing, shield, guard, evasion, and parry.