On the Stream page, you'll notice a large '+' button on the bottom right of your screen. When you click this you'll be given four options for ways you can post things to your Class.
The first option is to Create an Announcement. An announcement allows you to share different types of files with your students, such as a word document, PDF, image or video, as well as website links and YouTube videos.
This is a great way to share out a powerpoint file you have been presenting from in class, or to post a link to a relevant website for further reading.