Tawheed means to believe in the uniqueness of God. It comes from the word Wahdat which means Unity. The one who believes in the uniqueness of God, is called Mowahhid ("موحد") .
Shirk means to believe that God has one or more partners. The one who believes that God has partners, is called Moshrik ("مشرک"). For example the ones who believe in the God of War, the God of Wind or something like that, are called Moshrik.
Shirk is associating partners with Allah or it means polytheism. Whereas, tawheed means to believe that there is only one god which is also known as monotheism.