Community

 There are two different assassin classes - the Assassin and the Executioner. The Assassin is built on the original framework of the 4E classes, while the Executioner is built on an Essentials frame.
They do things in different ways - they first is someone who's sold part of their soul to the Shadowfell for access to shadow magic and the second is a completely martial class who basically sneaks around and does the stabbity thing.

Basically, the orinal assassin, or Ossassin, is an arcane, magical killer, using shadowmagic and teleportation along with weapons. The Essassin, or Essentials assassin, is a martial base, using completely physical attacks and poisons.

First of all, both types can use poison.  Anyone can use poisons, and both assassins are good at making the most of them.

The Essentials assassin (Executioner) has an extra set of poisons that it gets instead of daily powers.  These special poisons can only be used once a day, and cost nothing to make - basically they are daily powers in poison form.  Only Executioners get access to these poisons, although an original assassin can take a feat to swap a daily power for a poison.  The Executioner also has all of it's encounter powers rolled into one big pile of extra damage that they can add onto one hit per encounter.  All other times you'll be using at-wills.

The original Assassin has "normal" encounter and daily powers, but it can still apply regular poisons to its weapon, same as a rogue or fighter or anyone could.  But those poisons are consumable items they have to buy.  Some of the encounter and daily powers have the poison keyword, so think of them as having poison already built in.

The two types work very differently, so you should read up on both before you decide which one to play.