Bits and Bytes both measure amounts of data. However, they are typically used in two different contexts.
Computer memory is usually measured in megabytes or gigabytes. This tells how much information your computer can store.
The speed of a modem (a device that connects two computers over a telephone line) is measured in bits per second, or bps. This tells how much information can be sent in a second.
These slides show terms and division of bits and bytes