Could someone tell me what the difference is between the birth control pills prescribed to someone on a 21 day cycle as opposed to a 28 day cycle? Thanks.

For the combined pill:
21 day: the packet contains 21 "active" (horomone-containing) pills. After taking all of these, the user takes no pills for seven days. She then starts a new pack of pills.

28 day: the packet contains 21 "active" pills, and 7 "inactive" (placebo ie non-hormone containing) pills. The user takes all 21 active pills, followed by the 7 inactive pills. She then starts a new pack of pills.

The withdrawl bleed (period, but called "withdrawl bleed" as it is caused by the body going through withdrawl from the hormones when they are stopped) occurs during the seven pill-free days or during the seven placebo-pill days, depending on if the user is taking 21 day pills or 28 day pills.

Also, the mini-pill is in 28 day packets. The mini-pill contains only progestin, whereas the combined pill is both estrogen and a progestin. With the mini-pill (or "progestin only pill") ALL 28 pills are active. No break is taken, and the period generally arrives during the fourth week of pills.