Here's what the outside of a typical smoke detector looks like.
When you take off the cover, you find that a smoke detector is pretty simple. This one consists of a printed circuit board (see the article Inside a Remote Control for an explanation of printed circuit boards), an ionization chamber (the silver cylinder toward the top right in the following picture) and an electronic horn (the brass cylinder toward the bottom right in the following picture):
Here is a close-up of the board:
and the underside of the board:
The ionization chamber is an aluminum can containing the ionization source. You can see that the can has slots in it to allow air flow. The can itself acts as the negative plate of the ionization chamber.
Underneath the can is a ceramic holder that contains the positive plate of the ionization chamber. Under that plate is the ionization source, WHICH YOU WANT TO BE SURE NOT TO DISTURB.
For more information on smoke detectors and related topics, check out the links below.