The vulva and vagina have a "normal flora" of "friendly" microorganisms (including the lactobacillus which turns milk into yogurt?) which help to keep the area healthy. When the normal balance is disturbed, an infection can result.
The generic term for infection of the vagina is vaginitis. Symptoms may include an unusual or unpleasant discharge, itching, or pain during intercourse or at other times, but vaginal infections may also be present without any noticeable symptoms.
See also Sexually transmitted diseases, Pelvic inflammatory disease