There is a fairly simple explanation here: http://www.paragon-press.com/lens/lenchart.htm

To summarize from that site:

Simply put, the focal length of a lens is the distance from the lens to the sensor, when focused on a subject at infinity. To focus on something closer than infinity, the lens is moved farther away from the sensor.

Focal length and focus distance are two different things.

Focal length controls the viewing angle, essentially meaning how much of the scene the camera can see. A large focal length means the camera sees only a narrow view of the world, which makes faraway objects look bigger. A small focal length, on the other hand, means the camera can see a wide view of the world. Objects appear smaller because lots of things get squeezed into the picture.

Focus distance is controlled by moving the lens further away from the sensor, so that the light rays from a single point on an object nearby converge to form a point of light on the sensor. If a 55mm lens were 55mm from the sensor, only objects infinitely or very far away would be in focus. To bring a scene into focus, the lens must be moved away from the sensor until all the rays of light converge to form distinct points. This is why almost every lens can focus on distant objects, but macro lenses (which focus on very close objects) are more expensive.

For additional reading, check out: http://www.howstuffworks.com/camera.htm