The bit is the smallest unit of measurement on a computer. There is almost always eight bits in one byte. Very often these two terms are confused with each other. To make it even more confusing, the abbreviation for bit is a lowercase b, while byte is abbreviated with an uppercase B.
Most of the time, a kilobyte refers to 1024 bytes. A megabyte is 1024 kilobytes, and a gigabyte is 1024 megabytes. The next level would be terabytes, or 1024 gigabytes, but hard drives really haven't gotten past that at this point in time.
However, when you are talking about the speed of an internet connection, it is almost always going to be referred to in bits, whether it is kilobits (Kb), megabits (Mb), or gigabits (Gb). So, if the speed of a connection is referred to in megabits, for example, it would be megabits per second, or Mbps. That means a 100 Mbps connection can transfer 100 megabits every second, which equals about 12.5 megabytes per second.