A toilet seat is a hinged unit consisting of a round or oval open seat, and usually a lid, which is bolted onto the bowl of a toilet used in a sitting position (as opposed to a squat toilet).
The seat can be either for a flush toilet or a dry toilet. A toilet seat consists of the seat itself, which may be contoured for the user to sit on, and the lid, which covers the toilet when it is not in use – the lid may be absent in some cases, particularly in public restrooms.
Toilet seats often have a lid. This lid is frequently left open. The combined toilet seat and lid may be kept in a closed position when a toilet is not in use, making it so—at a minimum—the lid must be raised prior to use.
It can be closed to prevent small items from falling in, to reduce odors, for aesthetic purposes or to provide a chair in the toilet room. (wikipedia.org)
These days most toilets feature a soft close toilet seat. This means that when you drop the seat down it falls in a gentle, controlled, silent fashion. No longer do you need to suffer the sleep shattering clanging of an old style toilet seat in the middle of the night.