What heroes do I pick on what maps?

I've been playing HotS for a couple weeks now, but just started to do unranked draft. However, it seems like everyone's speaking another language. During drafts, my team will talk about what heroes are viable, and (more often than not) how my pick ISN'T viable. I like the increased strategy in drafts, but for the life of me I cannot figure out what heroes to pick for maps. I'll pick Uther for one map and there's no problem, but on the next map it's a bad pick. I really don't know. Can anyone help? Or point me in the direction of anything that explains this?

And on the topic of lineups, what's the standard team composition look like? I know there should be a support, but people got upset when I picked Tassador while we had a Rehgar (I mean, really? Tass isn't much of a support). So... I'm thinking it's usually 1 support, 1 tank, 2 DPS?