Infant seats have bases that stay in the car, and the seat can be removed, while convertible seats stay in the car, and you just lift the baby in and out. You don't *need* an infant seat, but they're nice to have when dealing with a tiny baby, and they do give you a place to put the kid when you're out. We're still using one with a small 11-month-old.

Convertible seats are convertible because they can be front or rear-facing. Weight limits vary, but I'd choose a fairly high one, to avoid having to buy a new seat too soon. We have an Evenflo infant seat I like, that was given to us by friends whose daughter grew quickly, and hated it. It has a matching stroller which I find clunky to use. Not that you asked, but I'd stick with umbrella strollers or a mei tai-type carrier. So much less hassle. We also have a Graco (I think) convertible seat in my husband's car. I don't use it, so no reviews there. I think most car seats have washable, removable covers, but make sure you get one that does!

Baby Cheapskate tends to list car seat deals, among other things. Hope it's okay to post that site.