Question: what is the difference between an orc and a uruk hai?
Answer: Uruk-Hais are Orcs. Just a special breed, that is stronger, faster and can withstand sunlight. Although Treebeard speculates that Saruman created them by crossbreeding Orcs with men, there is no tangible evidence for that. Nonetheless your average Orc is bow-legged while Uruk-hais have straight legs.