A 'bit' is a 'binary digit', a small unit of computerized data. A bit is smaller than the size of a single typed character in an email. For the sake of ease, think of a bit as approximately the same size as a text character . A megabit, then, is approximately 1 million typed characters.
The Little 'b' vs. the Big 'B':
Yes, this is confusing. The little 'b' is for 'bit'. The capital 'B' is for 'Byte'. Hence, Mb and MB are abbreviations for the smaller vs. larger data sizes.
Example: My internet connection is 50 megabits-per-second download speed (50 Mbps)
This statement describes a connection speed of approximately 5 thousand pages of plain text in 1 second.
Example: My smartphone gets 4 megabits-per-second on my network.
This statement describes the cellphone connection speed as approximately 4 hundred pages of plain text in 1 second.
The Big Brother, 'Megabyte':
A Megabyte (MB) is 8 megabits combined. A megabyte is approximately 8 hundred pages of plain notepad text. Megabytes are commonly used to measure hard drive space and RAM memory.