The canvas is stretched tightly over a purpose built wooden frame, over the edge, and stapled to the back.
The edge is usually (in my case) painted in dark grey to make a neat finish to the painting.
The back is strung and is therefore ready to hang.
In my opinion this creates a contemporary look and doesn't need a conventional frame. However for those who prefer a classic frame it is easy to remove the painted canvas from the wooden frame and then put it into a conventional frame.