This is the difference between a 'magazine' and a 'clip'
Pop Quiz: Identify the clip/s in the image above.
If you pointed to the bullet-holder-thingy on the left, congratulations! You know your weapons. Or you're gifted at taking stupid tests. Either way, you've got a bright future here in America.
While people often confuse the terms, clips and magazines serve different purposes. The simple way to think of it is this: the magazine feeds the weapon whereas the clip feeds the magazine.
A magazine is what you're used to seeing in films or loading into your weapon or snuggling with at night.
Don't google "gun jokes" — it's depressing af. (Image of 9mm magazine via wiki user Scoo)
The magazine stores ammunition and feeds it to the weapon. It commonly holds rounds under spring pressure for fast loading into the chamber. The name comes from the French word for storehouse or storage place, but some guy in this forum says the word goes back to the time of Christ, and dammit, I believe him.
(I don't. I'm not saying Mike is a liar, I'm just saying I need to see some proof.)
Now, anyone who has loaded cartridges into a magazine can testify that it can be
a bitch labor intensive. A clip, therefore, holds cartridges together for easy loading into the magazine.
An M1 Garand en-bloc clip (left) compared to an SKS stripper clip (right).
If you want to dive a little deeper into it, this gent is all about them gunzzz:
Oh, and while we're on the subject. A cartridge goes into a gun (and contains the case, the projectile, the propellant, etc) and a bullet shoots out of it.