A solar cooker utilizes passive solar energy (solar energy that has not been converted to another form, such as electricity or mechanical energy, by a solar panel or Stirling engine) to cook food.
The basic parts of any solar cooker are a reflector, to direct most of the solar energy reaching its large surface to the food being heated, a container to hold the food, and insulation to prevent the heat from escaping too rapidly.
There are a lot of really cool plans for a solar cooker available at http://solarcooking.org/plans.htm, mostly built using items you have in the kitchen right now.
This is a basic plan I got from the Florida Solar Energy Center long ago:
Lid (Glass or acrylic, Lexan, etc.)
(One of the new plastic oven bags (Reynolds?)
might work, if weighted or secured in a frame)
| | | | Outer cardboard or
| | Cardboard box, | | wooden box
| | <-- foil lined -----> | | <----
| | _____o_____ | |
| | | | pot | |
| | |___________| | <--- Air space for
^ Spacer of some sort, to
prevent heat conducting out the bottom
The pot can be either glass or dark colored metal, such as cast iron. An old black cast iron skillet would probably be ideal, as it will not only absorb heat, but will continue to radiate it for a very long time. You may want to preheat a big iron pot before use.
The design of a solar cooker can be elegantly simple (or complex), and works anywhere that you have direct sunlight without continuous cloud cover.
Once you have the cooker built, you can try out some solar-cooked recipes... a listing of some can be found at http://solarcooking.org/recipes.
In general, when cooking anything large or with a large volume of liquid, such as a roast or soup, it may be a good idea to preheat it before placing it in the solar cooker, unless you're using a cooker with a large or parabolic reflector (as found with the other plans at solarcooking.org).