1,000 Bytes, but commonly refers to 1,024 Bytes which is actually called a KibiByte. However, this distinction has not been adopted in practice; with very near certainty, the term KiloByte is always used to refer to 1,024 Bytes.
In speech, it's often cut down to "K", ie pronounced "kay".
This is a very common unit. Many BinaryExecutables, libraries, configuration files, images and so on range from somewhere between half a KiloByte to a few MegaBytes.
See also KibiByte, MegaByte/MebiByte, GigaByte/GibiByte, TeraByte.
