The basic unit of a storage space is in bits. However, the physical computer technology practically sees everything in bytes. Each byte contains 8-bits. The unit symbol for bit is a lowercase 'b', while the unit symbol for byte is an uppercase 'B'.
It is generally it is more practical to talk about megabytes (MB). But many manufacturers today like to use megabits (Mb) to make their number look bigger. So you'll really have to be careful when comparing products.
For example, DSL and cable companies like to talk about 5 Mbps transfer rate into your home, which translates to 0.625 MBps after dividing by 8.