said by recneps :

To clarify his explanation:
KiB (kibibyte) and MiB (mebibyte)
are used to distinguish between "true" binary values (1024) and the "metric" (is it really metric?) values (1000's)

KiB = KB in terms of computer language (1024 bytes)
MiB = MB in terms of computer language (1024 kilobytes (of 1024 bytes each))

This is opposed to the now common (by hard drive manufacturers) use of (technically true) kilobyte ("1000 bytes") and megabyte ("1000000 bytes)
Much like the metric system (powers of 10)
In this system KB is not the same as KB above (1000 bytes vs 1024), so KiB is used to ensure there is no confusion.
(same with MiB, GiB, and so on)