What is the difference between a rectal ulcer and a hemorrhoid? one's a vein right? where do rectal ulcers lie? are they the same as stomach ulcers?

Very different : Hemorrhoids are dilated veins in the analysis canal. Rectal ulcers involve damage to the surface tissue often related to trauma (like from a piece of hard stool), infection or inflammatory disease(like crohn's or ulcerative colitis.) While rectal ulcers and gastric(stomach) ulcers are both superficial tissue damage, they tend to have very different causes.

Ulcer vs hermorrhoi: Hemorrhoid as you stated is an engorged dilated rectal vein. An ulcer is an erosion/abrasion on any mucosal surface like mouth, esophagus, stomach, intestine and rectum. The concept is the same the location may differ. rectal ulcers can lie anwhere in the rectum as they can be from infection ( herpes) or autoimmune disease ulcerative colitis