Sorry for the beginner's Q, but I didn't see a noob forum, so asking it here.

I currently serve 3 300x250 Adsense units on my site, but I'd like to replace the native Adsense units with those units (sorry, not sure what they're called) where you just drop in the 300x250 placeholder, and then you can manage which ad networks' med-rec ads get published from a single back-end. So a few rapid-fire Q's:

1) Just what are those multi-network ad services called? Is there a name for them?

2) What services are the biggest players in this space? (aka: where should I go first to check out various offerings?)

3) Do I have to sign up with all of the individual ad networks on my own first so that I can input my ad code in the one-stop-service's back-end? For instance, I want to serve some Amazon 300x250 ads...do I have to go and sign up as an Amazon affiliate first so that I have an affiliate code of my own, or do I just need to create an account with the multi-network ad service?

4) Do these multi-network services take any cut of my ad revenues (in exchange, I suppose, for the service they're providing)? IOW, is there any difference between the native Adsense units I currently publish, and the Adsense units that would get published via the multi-network service?

5) Any other hidden downsides to going this route?

Thanks for any help.