If I try to explain it in my own words it'll just come out as quite a lot of swearing, so here's the official NHS line:-
Endometriosis is a common condition in which small pieces of the womb lining (the endometrium) are found outside the womb. This could be in the fallopian tubes, ovaries, bladder, bowel, vagina or rectum.
Endometriosis commonly causes pain in the lower abdomen (tummy), pelvis or lower back. It may also lead to fertility problems (see Symptoms of endometriosis for more information). However, some women have few or no symptoms.
The cause of endometriosis is uncertain, but there are several theories.
Endometriosis affects around 2 million women in the UK. Most of them are diagnosed between the ages of 25 and 40.
There is no known cure for endometriosis. It is a chronic (long-term) condition that can cause pain, lack of energy, depression and fertility problems. However, symptoms can be managed and fertility improved with pain medication, hormone treatment or surgery, so that the condition does not interfere with your daily life.
After years of laparoscopies, being put through an early menopause for 3 months, pills & disciplinaries for absence from work I was told it was hysterectomy time or try to get pregnant, if I could, which was doubtful.
What do you know? I got pregnant very quickly and was given nearly 3 years of respite from pain until I stopped breastfeeding and the dreaded hormones kicked in again.
I've since discovered that my adhesions are so widespread that a hysterectomy would not bring me full relief of symptoms anyway. Stage 4 is the final stage and a lot of women have it much much worse than me.
This time round it's back in my life but I'm not going down the NHS route until it's totally necessary.