Megabit and Megabyte are quite different. One megabyte is equal to 8 megabits; this is because 8 bits is equal to a byte. Megabyte are mostly used when talking about the size of a document or you are talking about the storage capacity of a particular device. That's when you see people saying my device is having the internal storage of 250MB.
There are some devices that have their capacity measured only in Megabyte (MB). Megabyte is also used to describe the amount of data someone has that can be used to surf the internet. While megabit represent 1000000 bits of information that can be recognized and processed by computer. The basic way of representing data on computer is bit. Which means when you have anything written in this way megabits per second, it means 1000000 of bits of data are being transferred per second. Megabits is also used when talking about the speed of internet connection.