Please help! what is the difference between reiter's syndrome and ankylosing spondilitis?

Very similar: These are both associated with the same genetic predisposition and are really just variants of the same disease. They are lumped into a disease category known as seronegative spondyloarthropathies so there is major overlap. If your issues are confined to the spine you can call it ank spondy. It you have urinary issues and a peripheral arthritis without back pain you can call it reiters.