A tampon is a piece of material a girl inserts in her vagina during her period to absorb the menstrual blood, preventing it from running out. The vaginal walls hold it in place.
A string is attached to the tampon. This string remains outside the vagina, so you’ll be able to remove the tampon by pulling the string.
You can buy them with or without applicator. The applicator is a small plastic tube that works like a plunger.
Tampons come in various sizes. This has nothing to do with the size of your vagina, but with how much blood you loose. With a light flow you use a small tampon and with a heavy flow a big one.
Using a tampon that is more absorbent (bigger) than you need can cause irritation and dryness of the vagina, making removal difficult. In rare cases it leads to toxic shock syndrome.
At the start of your period the flow is normally heavier than in the end, making it necessary to change tampons more often.

How to insert a tampon

I hate sanitary napkins but am afraid of tampons. What can I do?
When I insert an tampon it gets stuck just past the opening. Can you give me some advice?
The string came off when I tried to pull the tampon out. What should I do?