There are two main types of Foot Joints. The first one is a standard C foot. This Foot joint has the Eb key, the C# key, and the C Key. All student line flutes come with this standard footjoint.
Professional and intermediate flutes have the B foot joint. The B foot joint has an extra key on the end of it. This extra key extends the range of the flute down to a B3. When playing with the B foot joint, the lower register comes out much better. It also helps with intonation in the third register. This thrid key on the end is sometimes known as the "Gizmo Key."
As a beginner, you should start out on the C foot joint. Flutes with just the C foot joint are cheaper than ones with the B foot joint. As you continue to play, and you wish to upgrade to a better instrument, that's when you will want to look into purchasing a flute with the B foot joint. Personally, I feel that you should upgrade after three to four years of playing. If you need help buying a flute, check out the flute buying section.