Answer: Shirk is dedicating any sort of worship to other than Allah Almighty.
Its types are:
Major Shirk, like supplicating, prostrating, or slaughtering a sacrifice for other than Allah Almighty.
Minor shirk, like swearing by other than Allah, using amulets, which are things worn or hung to bring about benefit or ward off harm, and slight ostentation, such as praying in a perfect manner while people are looking.