Nmbr5Usually when people say "ear bud" they're referring to something that sits loosely on your outer ear, like a little speaker that you're resting there. An IEM is inserted deeper into the ear and usually has silicone or foam tips (unless it's a custom molded to your ear).
In practice, earbuds are more like open-back headphones, in that they don't isolate and usually have a wider soundstage. IEMs are more like closed-back headphones, because they do block out noise and usually have a narrower soundstage.