1,000 KiloBytes, but commonly refers to 1,024 KibiBytes which is actually called a MebiByte. However, this distinction has not been adopted in practice, so it's not always clear what exact capacity is meant by a particular mention of MegaByte.
In speech, it's often cut down to "meg".
Traditionally in computer jargon, since computers use base 2 rather than base 10 for numbers, Kilo- means 210 (==1024) rather than the typical 1000. However, now that computers are used everywhere, this has confused many people who assume it implies base10; hence the proposal to use the Kibi- prefix for the base2 version. Similarly for Mega- traditionally meaning 220 (==1,048,576) rather than 1,000,000.
This is a very common unit. The majority of sets of data dealt with in contemporary computing range from somewhere between half a MegaByte to about 50. 400-600 MegaBytes is a common size for MultiMedia data, owing to the fact that that's roughly the capacity of CDs.
See also KiloByte/KibiByte, MebiByte, GigaByte/GibiByte, TeraByte.
