Most brands of combination pills come in packs of 21. You take one tablet every day. After 3 weeks you have a 7 day break in which you have your period. After this pill free week you start a new pack. During the whole period you are protected against pregnancy.
There also exist 28-day packs, adding 7 ‘reminder’ pills. They contain no hormones, but help to maintain the daily routine during the 7 days between active pill use.
The pills contain a combination of an estrogen and progestin. These are similar to the hormones found in your body. They work in three ways to prevent pregnancy.
Combination pills vary from one brand to the next. They normally use the same kind of estrogen: ethinylestradiol. What differs is the amount of estrogen and the kind of progestin that is used. If you have side effects you can change to another brand, with a different kind of progestin.