In Short: FAM successfully identifies a woman's fertile period by relying on a few physical signs: menstruation, cervical fluid, waking temperature, and characteristics of the cervix. Because it relies on signs and not day-counting, FAM may be used by women with both regular and irregular cycles.
Fertility Awareness works by analyzing just a few signs in the woman's body. In a nutshell, a woman's fertility cycle operates according to a predictable rhythm with specific signs that show where in the cycle she is. An obvious example is menstruation. Less obvious, but just as trustworthy, are changes in waking temperature and the quality of cervical fluid. The position and texture of the cervix is also indicative of fertility, but it may be harder to interpret and is not used in FAM other than as a corroboration of temperature and cervical fluid signs.
A woman's ovum (egg) only lives for about 24 hours, but because the life span of the man's sperm is longer, the "window of fertility" is about 6 days. Under the best conditions, sperm may live in the uterus and fallopian tubes for a maximum of 6 days, although 2-3 days is more common. The goal of fertility awareness is to identify when this "window" will occur. Fortunately, the body gives a few clues to determine the start and end of this window.