The lining of the uterus is called the endometrium.  Each month, during a woman's cycle, the endometrium responds to hormones that tell it to thicken with blood.  This is to prepare it for a fertilized egg.  If no fertilized egg presents itself, the endometrium sheds itself during what we call a "period."  Endometriosis occurs when the endometrium is found outside of the uterus.  It can be found on the outer layer of the uterus, the ovaries, fallopian tubes, sometimes even in the lungs, although this is rare.  The endometrium in these sites reacts to the same hormones and goes through the same process.  However, when it is time to shed, it has no way to exit the body.  A woman with endometriosis often, although not always, feels pain in her pelvis when these "rogue" endometrium tissues swell and try to exit.  Afterall, they aren't where they are supposed to be (shame on them!).  In more advanced stages of endometriosis, the tissue around the endometrium may scar and cause adhesions.  It is this scarring of the tissue that may cause infertility, depending on where the adhesions are and how severe they are.
40-60% of women with endo do not experience infertility, so don't freak out yet!

No one really knows.  However, there are several theories, none of which have been proven.  Here are two of the more popular.

Sampson's Theory
This theory is the most common, although it has many flaws.  It is also called the retrograde theory. The idea here is that some of the menstrual flow flows backwards up through the fallopian tubes and into the pelvic region.  The endometrium implants itself and begins to grow.

Dioxins
Another theory is that there is a relation between dioxins and endometriosis.  Dioxins are chemicals most often found in tampons.  They are used to give the tampons the bleached, clean look.  Those who believe that dioxins and endo are related advocate that you use other methods than tampons during menstruation.  If you definitely prefer tampons, try to use those that are 100% cotton.  I believe Tampax has a line of such tampons.

I've heard various statistics on this.  I've heard 1 in 4 will have it, I've also heard 1 in 7.  Although there is no proof the endo is hereditary, those who have a first degree relative with endo are at a higher risk.  Endo is not discriminatory.  At one time it was thought that only upper-class caucasian women got it.  Later it was called "the working woman's disease" (because doctors and employers thought they were trying to get out of work by complaining of pain).  We now know, though, that any woman of child-bearing age can develop endo.

Not every woman will experience all of these symptoms. Some will experience some, others more, some none at all.
 

The only accurate way to diagnose endo is through a surgical procedure called laparascopy.  An ultra-sound will not show endometrial tissue.  Believe me, I had two ultrasounds and they showed nada.

Laparoscopy is a simple out-patient procedure.  You will be put under a general anasthesia. Your 
abdomen will be inflated with a harmless gas.  This is to separate the reproductive organs and the intestines so the doctor has a better view of what's in you (you want to make sure he can see it all, don't you?) The doctor will insert an instrument called a laparascope in a small incision in your navel. The laparascope is a microscope that transmits light.  Other instruments will be inserted in another incision above or at the pubic line.  These instruments are used to probe the organs and to remove endometrial tissue.  Your doctor may choose to cauterize the endo or remove it with a laser.  Make sure you know before your surgery what his plans are if he finds endo.

After surgery you will be allowed to recover in a room for those having out-patient surgery.  You will be allowed to go home later that day.  For a better idea of what to expect before, during and after surgery, see the surgery tips page.

Endo is categorized into four stages.  These are currently under review, but for now they are:
Stage I - minimal, Stage II - mild, Stage III - moderate, Stage IV - severe.  Several factors are used to determine the stage of endo. During your post-op appointment, your doctor will tell you what stage endo is/was present.

First you should know that there is no cure for endo, only treatment.  If you doctor has removed the endo, it may never come back again, but there's no guarantee.

Treatments for endo are varied.  Your treatment will depend on the stage you are diagnosed at and other factors.  Treatments include:
 

Your doctor may suggest that becoming pregnant is a "cure".  Although your symptoms may be relieved by the pause in your menstruation and be lessened afterwards, again, there's no guarantee that this will "cure" endo.  Also, a hysterectomy is not a cure for endo.  Although the uterus may be gone, the endometria can still be present.  Many women who have had a hysterectomy have had endo recur.  Be concerned if your doctor immediately suggests a hysterectomy as treatment. (This happened to my best friend when she was 19.  Fortunately she didn't listen and she's now with another doctor who's a little smarter.)