Maplestory Thief Introduction

Thieves are a very popular but expensive class. They have their likenesses with archers and warriors. Though probably being the most expensive to handle, they gain the most mesos in the process after magicians. They excel at killing enemies quickly, and thus gain lots of mesos in the process. The downside is that the Thief class demands precision when allocating stats, as they can be screwed up very easily.
There are two types of thieves: Assassins and Bandits. Assassins are quick ranged attackers. They are quite Meso efficient and have good damage potential, but cost much more than bandits. Bandits are quick melee attackers and are quite powerful among the 2nd jobs. They aren't as Meso efficient as Assassins and do not have the benefit of ranged attack but make up for it in much greater raw power (after around level 90, there is no way that any assassin can catch up with Bandits' damage), although dagger scrolls have a potential to cost quite a bit.

Pros

Both bandits and assassins get some cool and useful skills such as Savage Blow, Haste, Assaulter, Triple Throw and Flash Jump.

They have the second highest avoidability and accuracy after archers.

Assassins can take out mobs efficiently with Avenger.

Assassins are extremely mobile and have good range.

Both are good at bossing at 4th job.

"DEXless" assassins can easily hit the highest damage of all classes between levels 20-70.

Because of their damage, Thieves are popular choices in Party Quests.

Cons
Average HP and MP, getting 2-hit-killed by monsters at high levels.

Assassins (especially "DEXless" ones) are comparatively unpopular due to their stereotyped KSing.

Very unstable damage until mastery in 2nd job.

Good stars and armours are extremely expensive. Claw and Dagger ATT scrolls are some of the most expensive in the game.

Equipment

If you are new to MapleStory or starting on a new server, you should consider skipping armor every five or so levels. Thief items are pricey, so save for future armor. Dexless thieves, Hybrid sins, and STR bandits cost more for scrolls, weapon, etc. Becoming one of these as your first character will be challenging to keep up with buying armor and weapons on your own. If you like a challenge, definitely choose one of these paths.