What is the difference: iritis vs conjunctivitis?

Different tissues: Iritis is inflammation of the iris , the colored part inside the eye while conjunctivitis is inflammation of the conjunctiva, the thin transparent membrane over the white part of the eye.

Very different: Conjunctivitis is an infection, usually viral, of the mucous membrane of the front surface of the eye, the conjunctiva. It is self-limited and not associated with anything else. Iritis is an inflammation involving the iris and anterior chamber inside of the eye. This is a much more serious problem that be associated with diseases elsewhere in the body.