Similar to the kilobyte, a megabyte is a data size unit, used with relation to data storage and processing, as well as data transmission.
A block of 1024 successive kilobytes make up one megabyte, with each kilobyte having 1024 bytes and every byte being made up of eight bits.
Just like with the relation of bytes to kilobytes, the number of 1024 kilobytes for a megabyte is a result of the binary nature of the bits and bytes (2 to the power of 10 equals 1024).
Since one megabyte has 1.024 kilobytes, we get:
1 megabyte = 1.024 x 1.024 bytes = 1.048.576 bytes = 8.388.608 bits
See the page about bytes for more detailed information.