The PC muscle, which stands for pubococcygeus muscles, is actually a sling of muscles that support the pelvic floor and surround the internal genitalia. These muscles are involved in urination (when you stop yourself from peeing in mid-stream, youre using your PC muscles). Both men and women have PC muslces.
The PC muscles are also involved in orgasm, and many women find that doing regular exercises to strengthen the muscles changes the way their sexual response feels. Exercises that strengthen the PC muscles are usually referred to as Kegel exercises.
Like any muscles in the body, the PC muscles can become weak for a variety of reasons (for women, everything from lack of use to childbirth to radiation treatments for cancer can cause severlely weakend PC muscles).