A onesie is the traditional baby outfit that looks like a shirt but snaps in the crotch. A few companies have started making onesies that also snap up the front so you don't have to pull anything over baby's head (a lot of babies HATE that) and they are calling them all kinds of things from "Easy On Onesies" to "Undershirts"......I've even seen "Tantrum Free Tee"
But there is also the more traditional shirt that you can put on a baby that looks just like a regular shirt only in baby's size. These can either pull over the head or snap up the front (usually called side-snapping shirts or Kimonos). These are great for baby IMO. When I was taking care of my niece, we almost never used onesies unless it was part of a cute outfit - they were just more pain than anything. The snapping shirts were much easier to deal with (no fussing with them at diaper changes, no pulling over baby's head, etc) and they were great when her umbilical stump was healing because they are not tight around the middle like a onesie.
Rompers or coveralls are like sleeper only without feet (and some are more like shorts than pants). These are nice because it's all one piece so if you go somewhere and make a mess, it's an easy change. But they're a pain because there are like 8 million snaps to do up. With my niece, half the time when my brother dropped her off with me in the AM, she would be wearing a sleeper or romper that was snapped from the neck to the belly button and then hanging off like tails on a tuxedo for the rest because someone got lazy and stopped snapping
Personally, for my daughter (other than gifts) I went for snapping T's and dresses that don't pull over baby's head with little bloomers or shorts. To sleep in, I opted for sleepsacks and gowns - all easy access, no 800 snaps to do up in the middle of the night. The only thing I would change if I was having a boy is that I wouldn't put him in dresses

It would just be outfits that are a T (preferably one that is easy to get on and off) and shorts or pants.