Breakthrough bleeding is characterized by very light bleeding between periods that does not require a tampon or sanitary napkin and is usually red or reddish-brown in color. It is also referred to as "spotting," according to Wikipedia.
Breakthrough bleeding occurs due to hormonal fluctuations, whether from beginning or switching birth control pills or from the advent of pregnancy. While breakthrough bleeding can be very lengthy, tapering off after one or two cycles, it is not dangerous to a woman's health and resolves itself spontaneously. According to No Period, breakthrough bleeding can also be affected by smoking, drinking alcohol and experiencing stress. Wikipedia notes that persistent breakthrough bleeding combined with amenorrhea, a condition in which menstrual bleeding stops entirely, may indicate a thin and underdeveloped endometrium.
If breakthrough bleeding lasts for longer than six months, consult a physician and keep a menstrual diary in order to discover triggers. No Period warns against stopping hormonal birth control due to breakthrough bleeding and advises taking pills as consistently as possible in order to eliminate spotting. Breakthrough bleeding can also occur with the use of an IUD. If breakthrough bleeding fails to resolve itself, doctors may recommend switching to a non-hormonal method of birth control or to a different pill formulation.