This is how I understand it, and I was on them for a few years. You take a pill every day. The hormones (estrogen, progesterone) in the pill trick your body into thinking you're pregnant. This is why some women have side effects of weight gain and nausea and sore breasts, among other things. If your body thinks you're pregnant, it won't trigger your ovaries to release any eggs, so you can't get pregnant. Then for 7 days every month, you stop taking the pills, or take a sugar pill and your body suddenly realizes that you're not pregnant, so it tells your uterus to shed the lining, and you get a period. It's usually lighter with less cramping (that's why some doctors prescribe the pill when you have heavy, bad periods) because in addition to inhibiting ovulation, the pill also lessens the lining build up in your uterus. So, if you forget a pill and your body does ovulate, it's less likely that you'll get pregnant because the lining is just not adequate to support the fertalized egg.
I've read that the pill is about 98% effective. I believe the 2% failure rate is due to women forgetting and having 'breakthrough' ovulation. I have heard of the odd situation where a woman was on the pill, took it regularly, and still got pregnant, but I believe that's quite rare. It often takes a few tries for your doctor to get the right dosage to overcome your body's natural cycles and I think those rare cases are when women aren't taking the right dosage.
Just so you're aware, there is some concern about the pill being an abortifacient. Many believe that since the pill alters the lining of your uterus, and if an egg is accidently released (perhaps due to forgetting or otherwise) then it can't (or will have difficulty) implant/ing. Depending on your view of conception/life, some believe that this means the baby is 'aborted', since the pill has created a hostile environment. Just something to consider if you're sensitive to such things.

Hope this helps!

~prsmama

There are 2 types of chronic sinusitis:
1. Non-allergic, infective, and
2. Allergic complicated with infection.
Non-allergic, infective chronic sinusitis is due to frequently repeated and inadequately treated acute sinusitis. It manifests with a headache in the frontal region, nasal obstruction, nasal discharge and loss of the sense of smell. In cases of non-allergic, infective chronic sinusitis there may also be present symptoms including: cough, dry throat, mucous secretion in the throat when you stand up in the morning, etc.
Allergic chronic sinusitis has an unknown cause but it is always complicated with infection because of drainage of the disturbed sinuses. It has the same symptoms as not-allergic sinusitis plus some vasomotor reactions.
Diagnostic procedures for both types of chronic sinusitis are anterior rhinoscopy, X-ray (Hirtz) of the sinuses, CT-scan, diagnostic punition and skin tests for hypersensitivity.
Therapy and treatment of chronic sinusitis includes: antibiotics; local decongestives; anti-allergic drugs and procedures (antihystaminics, desensibilisation), punition, and surgery (polypectomia, resection of the nasal septum, trepanation etc.).