A megabyte is a collection of about 1,000,000 bytes, which is around 1,000 kilobytes.
In the metric system, mega is a prefix for 1,000,000, or 106. So in some cases, a megabyte is defined as 1,000,000 bytes.
But because computers work in binary, computer scientists usually define a megabyte as 220, or 1,048,576 bytes. This works out to 1,024 kilobytes. For programmers, the term "mega" rose out of convenience, as 1,048,576 bytes is close to 1,000,000.
The abbreviation for megabyte is mB in the metric system (1,000,000 bytes), or MB in the binary system (1,048,576 bytes).
Some groups have proposed the name mebibyte for a unit of 1,048,576 bytes to prevent confusion. The abbreviation for mebibyte is MiB.