This means that the platter is doing 7200 rounds per minute. More recent computer hard drives can now do up to 15,000rpms. As the platters go round and round the head reads it and processes the information which then proceeds to the connector which connects to the computer.
External drives are very useful for backing up your computer and for transfering files to two locations. External drives are also very portable and there are even smaller that use a computer hard drive that is very similar to a laptop hard drive.