The birth control pill work by stopping sperm from joining with an egg.When sperm joins with an egg,it's called fetrilization.In addition,it contains estrogen and progesterone safety stop your ovaries from producing a mature egg.No ovulation meas there is no egg for sperm to fetrilize,so pregnancy can't happen.
I use 28 days packs one.Take active pills for 21 days,and inactive pills for 7 days.I'll bleed on inactive week.
-- Why does it need rest period? --
While you are taking pills, your ovaries stop working.In order to keep the ovaries functioning normally,they most be restarted periodically.Even if you do not take pill during the rest period,ovulation will not occur because the egg has not fully grown.The egg will not suddenly grow larger and ovulate within 7 days.