A Faraday cage is simply a closed grid of metal (or some other electrical conductor). Electricity will flow around the outside of the cage, protecting what's inside. For example, a car could be considered a Faraday cage in a lightning storm, because if the car got hit by lightning, the electricity would flow through the metal body of the car, and you would be safe sitting inside.

Faraday cages are also used to confine or repel electromagnetic waves, like radio, cell phones, TV, or microwaves. For a Faraday cage to be useful, the openings in the metal need to be smaller than the wavelength of the electromagnetic wave. This is why your cell phone works in your car, with such big windows, but it won't work if you set it in your microwave oven and close the door. If you look closely at the inside of your microwave oven door, you'll see that the door is covered with a solid sheet of metal with a lot of tiny holes. The holes are much smaller than the wavelength of either microwaves (about 12 cm) or cell phone signals (3-30 cm).

A Faraday cage works both ways: keeping electromagnetic waves in, or keeping them out. If your cell phone *does* work in your microwave oven, have someone check the microwave, because it's probably leaking!

One important point with Faraday cages is that you cannot allow any gaps: a long slit is often as bad as a big hole. This is why it's important that your microwave oven door closes tightly. If you look closely at videos of people playing with big Tesla coils

So... the type of Faraday cage you want to build depends on what you want to do with it. It could be as simple as bending and welding together thin bars of steel in a dense mesh, or as complicated as dense chain mail body armor.

A Faraday cage is a container made of conducting material (e.g. metal) surrounding an object of interest, such as an electrical system. A simple metal box is a Faraday cage. The conducting metal absorbs any electric fields that might otherwise influence the system inside of it.

A Faraday cage is a closed container (a room or a box) with a fine meshwork of metal surrounding it. This metal acts as a conductor, and blocks electric fields from penetrating inside the container (or out of it!). When an electric field hits a Faraday cage, it forms a current in the metal which reorders the electrons in the metal until the charge is neutralized, shielding the inside from the outside. If the mesh is the proper size, it can also prevent certain kinds of electromagnetic radiation from going in our out.

You probably have a Faraday cage in your kitchen already-- Microwave shave a Faraday cage built into them so that the microwave radiation they produce doesn't leak out and cook you as you watch your food bake. In the microwave's window you can see the fine mesh work of metal filaments. This behavior also explains why cell phones sometimes lose their signal in buildings of elevators-- the metal of the elevator or the supporting metal struts in the concrete of the building are forming a Faraday cage that blocks the radio signals of the phone.