Ortho Evra is one of the newer methods of delivering birth contol hormones. Basically, the hormones are delivered through a time-release topical patch, and the are absorbed directly through the skin (this is refered to as transdermal administration).
Basics
"The Patch" has some advantages over the traditional pill in that it is applied once a week for three consecutive weeks, followed by a week off. This means much less hassle and confusion surrounding usage than, say, the pill.
The patch itself is a thin, square patch that closely resembles a nicotine patch. In fact, Ortho Evra is quite similar to nicotine patches in that it delivers its contents using the exact same principles. It is also completely waterproof (so they claim) and can be used while swimming, showering, etc.
Attachment
Ortho Evra's manufactures claim to have invented an adhesive which they stand by to stick for an entire week without problem (in my experience this has been true, though my girlfriend has experienced some shifting and occasional lifting of the plastic edges of the patch, but it has never fallen off or even come very close), though a small percentage of women (according to their clinical tests, less than 2%) tested did experience detachment of the patch. This is not necessarily problematic, though, because as long as the patch is replaced within 24 hours, the cycle can continue as usual without interruption.
How Ortho Evra Prevents Pregnancy
The drugs/hormones contained within the patch are basically the same time tested combinations used in "the pill" for years. The potential side effects are the same as well, such as nausea, menstrual cramps, breast tenderness or enlargement, weight gain, sex drive changes, etc (see this w/u for a more complete list of side effects. Also: my ex-girlfriend insisted I mention that some women may experience breast enlargement (which she found to be nuisance, but I enjoyed)).
Usage
The usage cycle is based on a 28 day schedule, beginning on the first day a woman starts the patch. The start day is variable either being every Sunday, or the first day of her period. Beginning with the first day, the patch is worn for one week, then taken off and replaced on the "switch day". Three patches are to be worn consecutively in this manner, and then removed for the week of her period. At the end of the fourth week, on the switch day, the cycle begins anew.