You've likely been using YouTube in the classroom for years. It's great for supplementing your lessons. There are a number of other ways you can use it, too.
If you're doing a flipped classroom or if you just don't like repeating yourself, then YouTube is a great tool for getting complicated directions to your students. Or sometimes not so complicated...
The above video was using a camera. But what if you want to screencast? You could definitely use screencasting software like Jing, but Google actually has you covered here as well.
Using Hangouts' integration into YouTube, you can screencast live on YouTube, saving the video for future viewing as well.
As I said above, you've likely been using YouTube for awhile. Do you use playlists? You can share those playlists, creating a series of videos for students to watch on their own. This is great for flipped classrooms, homework, and makeup work.