A thief is a fast warrior and a fast archer combined. They can be ranged or melee. However, they are not given enough skill points to max out both a melee skill and a ranged skill early in the game, so they must choose whether to be a Bandit or an Assassin. While quite different, these two kinds of thieves have a similar set of strengths. More or less, thieves are tired with Archers in the term of weapon defense. In the game, they have average HP and MP and they make up for their lack of HP through their high avoid-ability.
   Both Bandits and Assassins have massive speed bonuses in the second job, and have very high avoid-ability. In the game, thieves have the highest avoid-ability, mobility and have good accuracy too, but when compared with Bowmen, they will lose out because dexterity increases accuracy more than luck, which is the primary attribute to damage in thieves.
   But Bandits do not have a ranged attack. However, this is cured with Boomerang Step in fourth job, which is a semi-range skill, like third Job's Assaulter. But, Boomerang Step attacks up to multiple monsters, and it's a double assaulter, which means you go pass the monster once, then again back to where you are. They will have to walk up to enemies just like Warriors and slash them.
In addition, they hit multiple times, doing little damage each time, while Warriors generally do a one-hit massive attack. Therefore, Bandits don't get to Knockback larger monsters, such as Stone Golems. However, this allows them a more consistent attack, and works to their advantage when using Haste, a speed-boosting skill, to jump clear over the enemy and attack it at the same time. Another advantage of the Bandit is the speed it possesses. Assassins are Bowmen with only some Bowmen weaknesses. Assassins have extremely high mobility, as they are the only ranged class that can shoot in the air.
Assassins have slightly less range than Bowmen. They can shoot projectiles much faster than any other class in the game, from the first and second job only. The weaknesses of Assassins are that they have to carry large amounts of projectiles. Unlike Bowmen, they do not get a skill similar to "Soul Arrow" until fourth job. At lower levels, this can be quite a pain, because each job advancement adds a row of extra inventory space, and the skill in the second job, javelin mastery, allows more stars to be carried, hence limiting training times very greatly at low levels. Due to the popularity of this class leading to high demand, better stars are often very expensive to purchase. So, assassins naturally have fewer slots for their 'use' inventory.
A second weakness of the Assassin is that they do not carry the Knockback ability which the Archer class has. Also they do not have an Area of Effect skill like the Bowmen's Arrow Rain or Eruption. Avenger is the only mob attack they have which travels in a straight line like a Crossbowman's Iron Arrow. In spite of this weakness, Avenger does a bit greater damage than archer's AoE if accompanied with Shadow Partner. They do not have attention diverting skills such as puppet unlike the Bowmen, which makes the monster to target the assassin or bandit more often. They make up for this with their high avoid-ability. However, the damage an assassin deals is the most inconsistent almost throughout the game in that the primary attacking skill, Lucky Seven, does not allow javelin mastery to be stacked with it, unlike other classes where learning mastery increases the minimum attack. The inconsistency is further augmented with the 2nd job, when the assassin receives the skill "critical throw" which is similar to the archer's but with a higher probability of occurring.