Fibroids are benign muscle tumors which occur in the uterine muscles. Some women are very prone to these and develop quite large growths. Symptoms range from none to excessive pain and bleeding with ones period. Occasionally, they will interfere with pregnancy although this is more controversial. They should not progress to cancer.
Endometriosis is quite another problem.
Endometriosis represents some of the cells from the lining of the uterus migrating out the fallopian tubes into the abdominal cavity. Here they attach to different structure and continue to slowly grow and to act like endometrial cells from whence they came. This means that they wax and wane with ones period. They can produce excruciating pain in a cyclic manner wherever they have come to rest. This can literally be any organ of the abdomen.