All assassins must be of an evil alignment. They must also have 8 ranks in both Hide and Move Silently, two skills that are inherently useful for assassins (in real life, as well). Therefore, most rogues, monks, and other such characters would be fitting to take a few levels in Assassin. Assassins receive a d6 hit die, which is identical to that of the rogue. Indeed, the Assassin's class features are very similar to those of the rogue. Skill points received every level are 4 + the Intelligence modifier. All assassins receive a simple weapons and light armor proficiency, although most classes inherit those feats (except, among others, the monk).
At odd-numbered levels, assassins' Death Attack feat is upgraded by one die. The Death Attack is a special d6 sneak attack, with a chance of paralyzation. At 1st level, the Death Attack is at 1d6. At 3rd level, it is at 2d6, and so on. At even numbered levels, assassins receive a +1 Fortitude save vs. poison. At 2nd level, they have a +1 bonus. At 4th level, they have (overall) a +2 save, and so on.
At Level 2, assassins receive the Uncanny Dodge feat, which allows for the use of one's dexterity bonus to armor class if caught flat-footed. It is important for assassins to be dextrous. Also, they receive the spell Ghostly Visage, which is simply another way for assassins to be stealthy. The first of these is also received by various classes, including rogues, barbarians, and monks, and would be redundant if the character already had the feat.
At Level 5, the assassin receives Uncanny Dodge 2, which provides a +2 Reflex Save, as with training, assassins become more dextrous. Also, they receive the spell/skill Darkness, because these evil assassins can use stealthy methods to reach their goals.
At Level 9, they receive the spell Improved Invisibility, as assassins have the stealthy ability to become invisible.
Finally, at the pinnacle of their training, Level 10, rogues receive a final bonus of Uncanny Dodge 3, which provides an additional +1 to Reflex Save, added to Uncanny Dodge 2's effects.