In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy.

You know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons at the other terminal.

A capacitor is a much simpler device, and it cannot produce new electrons -- it only stores them. Like a battery, a capacitor has two terminals that connect to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper. It won't be a particularly good capacitor in terms of its storage capacity, but it will work.
In an electronic circuit, a capacitor is shown like this:

When you connect a capacitor to a battery, here's what happens:

The plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.

The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery.