A silhouette is an outline or general shape of something filled in with black and against a lighter background. Silhouettes are simple, but can add moodiness and mystery to an image.
To make a silhouetted photograph, your subject will need to be in front of your light source. You will then want to meter off the bright area of the background, making your subject (in the foreground) underexposed. You can back light with windows, a sunrise or sunset, or any other lighted source. It won't work if you have background light and another light source such as one from overhead or from a flash pointed towards your subject. With this being said, be sure to turn off your cameras flash.
If your subject comes out dark but not completely black, you can adjust your levels in your editing software to take the darks all the way to black.
Another thing to keep in mind when shooting silhouettes is the subject matter and the shapes that it makes. Since you're loosing the details that typically make a photo interesting, you'll be relying heavily on the positive/negative space aspect. Pay attention to how the shapes work with each other, and how identifiable they are without the details in them.