Most birth control pills are comprised of a combination of two hormones: estrogen and progestin, though some contain progestin only. These hormones prevent the ovaries from releasing eggs, so when spermies show up, there's nothing for them to fertilize. They also thicken cervical mucus (to make it harder for spermies to swim) and thin the uterine lining (so if on the off-chance something is fertilized, there's no soft warm place for it to latch on to and grow into a person.)
The Pill (of which there are a zillion brands to choose from) comes in monthly packs that cost between $15-$50. The packs usually contain 28 active pills and 7 placebo pills, which allow for a menstrual period each month. Some newer pills, though, come in packs without placebo pills, with the intention of reducing the number of periods women have each year, which I think is rad.
Birth control pills are taken once a day, and should be taken at the same time every day. This is super important, because each pill is formulated to work for 24 hours, and 24 hours only. A delay in replenishing the medication, even of just a few hours, can conceivably leave the body unprotected. If a pill is skipped, or more than a few hours late, it should be taken as soon as its remembered, and a backup method should be used for the rest of the month. It's a drag, but it's the reality.
There are considerable side effects of The Pill, though not all of them are considered negative. Happy side effects include a reduction in facial and body acne, the regulation of wacky menstrual cycles and the reduction in severity of menstrual cramps. They can also reduce the risk of Pelvic Inflammatory Disease, which is something I did not know! I love learning new things!
On the negative end of the side effect train are weight gain (about 5lb., on average), nausea and vomiting, mood alteration, reduction in sex drive and vaginal lubrication, bleeding between periods, and an increased risk for blood clots and stroke, especially in women who smoke cigarettes or have certain kinds of migraine headaches.
It's very important that all your healthcare providers knows you're taking birth control pills, as there are medications (like some antibiotics and anti-depressants) that can kill the efficacy of The Pill.
So, it's not right for everyone (I, for example, have experienced every single negative side effect of every pill I've ever tried), but birth control pills are crazy effective and very safe. If your bod can handle the side effects, and your brain can remember to take them on time, it's an excellent, safe and extremely popular choice for No Babies!