Most birth control pills contain an estrogen and a progestin. Those that contain estrogen use the same kind of estrogen (ethinyl estradiol), but the dose can vary. Several different types of progestin hormones are used in birth control pills. The strength of the estrogen, the type of progestin, and the dose of progestin all play a role in determining which side effects a particular birth control pill is likely (or unlikely) to cause.
Some progestins have androgenic activity (testosterone-like activity). These progestins are often thought to carry a higher risk for acne and excessive hair growth (a condition known as hirsutism). Other progestins have anti-androgenic (anti-testosterone) effects and are thought to be useful for treating acne or excessive hair growth. Some progestins may worsen water retention, while others may help relieve this problem. Differences in the estrogen content may affect menstrual bleeding tendencies, and many high-estrogen pills can increase the risk of blood clots.
I suppose these aren't so bad... as long as they're not happening to you. These are even the not-so-serious ones.
For the curious, here are some of the more serious ones:
Some birth control pill side effects, while occurring infrequently, are potentially serious and should be reported immediately to your healthcare provider. These include but are not limited to: