KB and kB refer to kilobyte - a unit of information or computer storage equal to either 1,024 bytes (210) or 1,000 bytes (103), depending on context.
MB refers to megabyte - a unit of information or computer storage equal to either 1,048,576 bytes (220) or 1,000,000 bytes (106), depending on context. A megabyte has approximately 1,000 (or 1,024 to be precise) kilobytes.
edit Difference between kB and kb, MB and Mb
kb is kilobits and kB is kilobytes. While MB is megabytes, Mb refers to Megabits. 1 byte has 8 bits. File sizes are usually expressed in KB and MB whereas kb and Mb are often used to express data transfer speeds (such as a 54 Mbps wireless router or 3G or 4G connection speeds).