Referring to Vince Young, The Disabled List has an explanation of what goes into a strained quad(s):
The quadriceps isn't a muscle, but a collection of muscles. These muscle run through your thigh and play a pivotal role in everything from walking to running to squatting. The muscles that are referred to as quads are as follows:
Vastus medialis is off to the side of your thigh.
Vastus lateralis is off the other side, opposite of the medialis.
Vastus intermedius runs right through the middle of the two other vastus.
Rectus femoris is the big dog of the quads. He runs through the thigh to the patella.
What are strained quads? It's a strain of any of the above muscles. A strain occurs when the muscle tears. It is usually smaller tears that occur.
The good news is, and this is something that should be fairly obvious, the treatment is the standard RICE - rest, ice, compression, and elephants.