How "locked down" are locked Android phones.

Hey there, going to be getting an Android phone soon, was hoping to look at developing some apps for it and stuff.

When I buy an Android phone and it's locked to a network, does this simply mean I can't use another network's sim cards, or would it also prevent me from loading my own custom apps, purchasing/installing those not in an approved Android store, stuff like that.

I'm assuming I can still do this stuff, just want to be sure.
If I can do this stuff without having to root/unlock the phone it'd be great, as I'm paranoid about the warranty and stuff.