How Earbuds Work
Taking broken things apart is a great way to find out how they work. If you're a young person, ask an adult first to make sure what you want to dismantle is really safe.
Here is my broken earbud and I've popped the back off it. You can see how the wires run up through the main case to the coil inside. We need two wires to make a circuit: one carries the current into the coil from the stereo; the other carries it back again.
Next, I've tipped the earbud over and popped the front cover off. The front cover is a plastic disc with holes in it to let the sound enter your ear. Just behind it, there's a very small cone. It's hardly cone-shaped, though: it's a flattish, transparent disc made of very thin and flexible plastic, and it's quite crinkly and crackly when you move it. You can just see the tiny metal coil (colored red) attached to it.
In summary, then, these are all the bits that make up your earbuds:
Back case: holds everything together. The wires run up through a hole at the bottom.
Front case: This is the part that faces into your ear. Sometimes it's covered with a little fabric pouch to keep it clean.
Seal: This rubbery circle clips the front case to the back case, holding the two together.
Wires: Carry signals from the stereo to the speaker.
Magnet: The permanent magnet at the back of the speaker. This is the heaviest part of an earbud and makes up the vast majority of its weight.
Coil: This becomes an electromagnet when electricity flows through it.
Transparent plastic cone: This makes the sound when it moves.