The definition for those depend on who you ask, but in this sub, IEMs usually refer to earphones that create a seal in the ear canal, usually with foam or silicone/rubber ear tips (see airpods pro). Earbuds are earphones that do not create a seal (see airpods).

In live music settings, IEMs are usually used for monitoring. On a stage, it is extremely hard to listen to what you are playing, so IEMs help musicians listen to themselves. They are usually comfortable, slim, and durable. Sound quality is usually NOT a priority: they just need to hear themselves, and that is a low bar. Some sound good, others not so much.

This sub is more oriented towards enthusiast-focused IEMs that offer a good listening experience. Durability and sound isolation also differs, but if you are just using them for listening pleasure only, its fine.

You can't really compare headphones and IEMs as they have different use cases, comfort, portability, etc.