KB is a written short form of kilobyte or kilobytes. A byte is a tiny unit of computer memory; a kilobyte is a thousand bytes — or, in an older computer habit that is still around, 1,024 bytes. People write KB for how big a file is: a short text file might be only a few KB; a photo is often thousands of KB, which we then write as MB instead. You will see KB on file sizes, email limits, and older computer screens. It is a writing shortcut, not something people say aloud much; in speech they usually say “kilobyte.” The letters KB can mean other things in other jobs, but in this use they only mean that small unit of computer size.