What is an 802.11ac router used for?

Im learning how to configure a wifi 802.11ac router. Whats the difference between a wifi connected one and an ethernet connected one?

I assume you are talking about ethernet connected to your router on one end and then going to some other device on other end for Internet connection? If yes then it will act normally as a wired connection and not wireless. 802.11ac is a wireless networking standard so its principles are only applicable for wireless connections.