Hurray, HrnyGoat is enlightened :).
Basically, 'b' = bit, whilst 'B' = byte (usually treated as eight bits nowadays, but this should really be called an ocet).
To elaborate needlessly further in a desperate attempt to get people to pay attention to standards that are a good eight years old:
1KB = 1 000B (SI style)
1KiB = 1 024B (2^10) (the 'i' for binary)
Etc, so 1MiB = 1 048 576B (2^20), and 1MB = 1 000 000B.
Makes a big difference with large files ? that's the main reason why your new hard disk always looks quite a bit smaller than you expected, the manufacturers always label capacity in GB, say, which are worth less (like US currency) compared to GiB (like Great British Pounds).