Ultram is just tramadol, an opioid-like drug. Ultracet is tramadol plus acetaminophen (Tylenol). And Darvocet is propoxyphene, a weak opioid, with acetaminophen. The "cet" part of the names means it has the Tylenol component. Like FioriCET has the acetaminophen in it, too.
Love and hugs,
Eddie