A mega byte is approximately 1 Million bytes of information.  The prefix indicates how large the number is.  The prefixes most commonly used originate from the metric system's powers of 10.  Most people are familiar with these prefixes is use with measurements of distance (meter, millimeter, kilometer etc...).  Each of these prefixes describes the power to which you raise 10.  For instance a kilometer indicates that 10 is raised to the 3rd power or 10³=1000.  Therefore 6 kilometers is 6x10³=6000 meters.  The following chart indicates the metric prefix, the appropriate power of 10 and the decimal equivalent:

When it comes to digital files things are a little different.  I say approximately 1 million bytes because of the way digital information is defined.  Digital files that the computer interprets are written in binary.  Which means that each bit (Binary Digit) of information has 2¹=2 possible values, 1 or 0.  Therefore, each bit can describe something in one of two ways either on (1) or off (0).  So in order to get higher levels of description other than on or off, extra bits are added in a string.  Once 8 bits are combined in a string they are called a byte.  A byte is comprised of 8 bits and can therefore describe something in 2⁸=256 ways.  One byte is the quantity of information required to describe one computer character such as the letter "K".  A 24 bit file indicates that something can be described  2²⁴=1,677,216 ways.  This is why you see 24bit color sometimes described as millions of colors.  So, back to the Megabyte, in binary terms a Megabyte (Mb) is actually 2²⁰=1,048,576 bytes, not 1,000,000 bytes as would be suggested by the metric prefix and a Gigabyte (Gb) is actually 2³⁰=1,073,741,824 bytes not 1,000,000,000.  This holds true for most digital storage and virtual memory values, but there is some cross usage of the terms for different storage devices and by different manufacturers, so you may not know exactly how many bytes any given device has in a Megabyte.  However, almost all other uses of the prefixes are in reference to the metric equivalent.  A Megapixel does equate to 1,000,000 pixels, an internet connection of 56 kbps is 56,000 bits per second.  Once again a confusing point regarding computers.  So here is the chart for the prefixes for digital files: