Actually only 12% of women have 28 day cycles, everyone else has longer, shorter, or irregular cycles. Here is a little diagram that illustrates the differing lengths of menstrual cycle, which can fluctuate from cycle to cycle in the same woman:
http://www.woomb.org/bom/cycles/index.html
And this little cartoon shows what the uterus, ovaries, cervix, and hormones are doing throughout the menstrual cycle:
http://www.woomb.org/bom/ovCycle/animation.html
Set the speed to SLOW so you can read what's happening at each phase of the cycle.