Nothing can get on a gun guy’s nerves faster than calling a magazine a clip. The simplest and most basic answer is that a magazine has some type of spring in it to force the loaded cartridges into a specific position to then be picked up by the firearms mechanisms.
A clip on the other hand is simply a device that holds a certain number of cartridges in a specific position. With a clip, there is no spring.
Confusion can sometime set in when you consider that a clip is often used to quickly load or fill a magazine. This can be the case with firearms such as the M1 Garand which have an internal magazine inside the rifle. That is to say the part with the spring that pushes the cartridges into place after each shot is permanently housed in the rifle. To facilitate a faster reload the user would have many clips of ammunition (enblocs in this case) at the ready to feed the internal magazine of the firearm.
A different example would be the AR-15 which has a detachable magazine that can be loaded either by hand or by 10-round clips (in this case known as stripper-clips) of ammunition. The detachable magazine is then reinserted into the rifle and can be fired.