I believe it is: ranged tank, melee tank, ranged DPS, melee DPS, and healerThe game AI handles which skills are used when, and so it is not as complex as Dragon Age Origins (thank goodness).
Then as if all this talk of Companions wasn't enough, a Developer Blog went up on SWTOR.com talking about...
Companion Characters Update - Part 1. Huh? I get the impression that they have just finished changing the companions, and are keen to talk about them. Most keen.
This blog post has some specifics on the type of companions, and it is good to hear that they give you ones which match your role.
Let’s take a deeper dive into one of the Smuggler’s companions, Corso Riggs, a heavily armored blaster specialist. When first acquired around level 7, Corso already has the tools necessary to be an effective tank.Smuggler being a ranged DPS, so a tank is a really useful companion to have.