Risk of Rain looks like a side-scrolling roguelite-shooter and in most regards that's what it is. It's also got a lot in common with ARPGs, in the way its characters' skills function and in how a newly unlocked character can completely change the way you approach the game. The latest update, version 1.2, is significant because it i) switches the game to a new engine and ii) adds two entirely new classes.
The patch brings a long list of fixes, but the first major change is the switch from Game Maker 8 to Game Maker: Studio. This brings with it performance gains, as explained when announced back in June: it's "actually-runs-the-final-level-at-60-fps better." Which sounds good. The switch also paves the way for the game's release on Mac and Linux, since the Studio version of Game Maker can export to those platforms.
As for the two new classes, they're the Chef and the Loader and both need to be unlocked as per any other character in the game. I won't say anymore for the sake of spoilers, but there's details of how to unlock them and what their new skills are over in this (not entirely complimentary) forum thread.
Don't know what Risk of Rain is? Read Adam's review. He liked it and I would probably like it, had I played.