Uruk-hai are fictional monsters larger and much stronger than orcs, that could resist the sunlight.
Within Tolkien's invented languages, the Elvish words for "orc" are derived from a root ruk referring to fear and horror, from which is derived an expanded form of the root, uruk. A noun *uruku is produced from the extended root. This eventually turns into Quenya urco, plural urqui. A related word *urko produces Sindarin orch, plural yrch. The Quenya words are said to be less specific in meaning than the Sindarin, meaning "bogey". For the specific creatures called yrch by the Sindar, the Quenya word orco, with plurals orcor and orqui, was created.
These orcs had similar names in other languages of Middle-earth: in Orkish uruk (restricted to the larger soldier-orcs), in the language of the Drúedain gorgûn, in Khuzdul rukhs, plural rakhâs, and in the language of Rohan and in the Common Speech orc.
In Tolkien's fictional legendary, the uruk-hai orcs were made by Sauron in the Third Age of Middle-earth in Mordor. Firstly they appeard in 2475, when the orcish army captured Osgiliath and Ithilia from Gondor.
In the Lord of the Rings (the novel) they were warriors of Sauron and Saruman too.
Books: Tolkien, J. R. R.: The Lord of the Rings. George Allen & Unwin, 1954-1955.