Hadarsi320They're called Custom In-Ear Monitors (CIEM), where the word "custom" means its shells are molded to the same shape as impressions of your ears. Besides that, I guess the main difference between any IEM and ear buds/phones is that IEMs are inserted into your ear canal rather than "hanging" outside your ear.
A driver simply refers to the component which converts electrical signals to sound we can hear. It is inside your typical earbud too.