Yep. Voila, Bio class for pre-med types is actually useful!
The two major hormones are Estrogen and Progesterone. FSH is Follicle Stimulating Hormone (stimulates one of the follicles in our ovary to mature to ovulation and start putting out its own estrogen and progesterone) and LH is Leutenizing Hormone, which causes the follicle to actually ovulate and go into the oviduct (beginning of the fallopian tubes) when it surges up.
The major BC pills/patches/rings/etc do one of two things: put out higher levels of both estrogen AND progesterone because that inhibits our hypothalamus from putting out hormones that cause us to eventually ovulate; OR they (like the Mirena) put out no estrogen and high levels of progesterone, which inhibits the anterior pituitary from putting out FSH and LH - thus no ovulation either.
Please note that during pregnancy, your body is producing mid-levels of both estrogen and progesterone (first from the corpeus luteum, which is what forms of the leftover follicle spot the egg came out of in the ovary, and then from the fetus itself) because mid-levels will also keep your lining thick and lustrous to better support a baby... and remember that this also keeps any other follicle from maturing to ovulation, since you only want one at a time.
So when you take pills that also put out mid-levels of estrogen/progesterone, they are imitating the state of pregnancy, keeping you from ovulating at all. So it's not really bad for you - studies have shown that women who have had children/been on BC have lower incidence of ovarian/uterine cancers than those who have not. And when you have your off week, the levels drop suddenly and that naturally causes you to shed your lining.
I hope that wasn't too much...
I can also tell you how the IUD actually works if you don't already know...