The Quads ("Quadriceps Femoris") includes 4 major muscles located on the front of the upper thigh:

• Rectus Femoris
  The most prominent of the 4 muscles.
• Vastus Lateralis
  On the lateral (outer) side of the thigh.
• Vastus Medialis (VMO)
  Located on the medial (inner) side of the thigh. This part of the Quad is extremely importance in rehabilitation of knee injuries, and notoriously hard to target.
• Vastus Intermedius
  Lies between the Vastus Lateralis, and Vastus Medialis muscles.

Each of the muscles in the Quad is an extensor (straightener) of the knee joint. The quad is one of the largest, thus strongest, muscle groups in the body.

There are many exercises to train the Quads. It is widely agreed that in an injury-free trainee, the most effective way to develop strength and size of the Quads is through a variety of Squatting movements, though it may sometimes be appropriate to include other exercises, such as Leg Extensions or the Seated Leg Press.