Origin and Archetype selection screen. Borrowed from HERE

The at you choose will determines what kind of powers you will have and what your major style of gameplay will be. Here is a quick summary of the typical role each AT (a section on powers follows):

Note: I KNOW already! Yes, you can play any character any way. If you try hard, you can make a super-tough defender and play them like a tank. However, most people don't and the game isn't going to make it easy for you. Therefore, I'm going to explain this how it's done normally. This is a guide for newer players after all.

Heroes

Tanker

Bring a friend. Hell bring EVERYONE.
I can take you all!

Their job is to hold Aggro and survive as best they can while the rest of the group beats the tar out of whatever enemies are around. In short, their job is being tough. A tank will take a bat to the knee, a gunshot to the chest, and an axe to the head while his player is off taking a potty break with no problems.