The confusion between these two terms is caused by people being lazy about how they write or talk about them. While they’re both about data, they’re quite different in how they are used. A Megabyte (MB) is a measurement of an amount of data, so your document or picture is so many MB. A Megabit is a shortening of Megabit per second (technically “Mb/s” often shortened to “Mbs” or just “Mb”) and is a measurement of the data transfer rate (colloquially “speed”) of a network.
A “byte” contains 8 “bits”, so an 8Mb/s network will transfer 1MB of data every second. Because people also confuse the capitalisation of the ‘b’, you can sometimes only work this out from context.
If you hear someone talking the size of document, or the total amount of data they’re allowed to download each month that will be in Megabytes (MB) or Gigabytes (GB) – 1GB equals 1000MB. If they are talking about a network speed, then they’ll be talking about Megabits per second (Mb/s). I have heard people say “That file is 25 Meg” and “my network is 25 Meg”. The first is Megabytes, the second is Megabits per second and they mean completely different things.