what are the things on the back of a canvas for?

They go in the corners of the canvas to tighten it. They will usually come like ten stuck together when you buy pre done canvases, but it's the same as the slanted rectangle wood pieces you may have seen before for the same purpose. The spikes are only there because they stuck together that way, they serve no other purpose.

Your a little vague with the description, but there are two possibilities.

The small metallic strip found at the top of a canvas, with a serrated lower edge, is, in fact, for hanging the canvas and allowing adjustments to make sure it hangs level.

You could also be talking about the wedges that fit into the joint of the frame and are used to tension the canvas.

I like to buy the canvas things and then frame them myself using home baked chandeliers bought from the ceiling & chrome shop located in East Argos. It is extremely important to get your lighting and quality of location in the lounge/dining room just nice. Spot on horrible if you don't as your neighbours will think your fashion is all lines and circles craap and probably think your abit of a horrible wiry modern nerd.

They are called "keys" and they are wedged into the slots at the intersections of two stretcher bars to expand the joint and tighten the canvas. Be careful when using them.

They are boards that the canvas was stretched onto, and yes, they do make hanging easier, but I also like painting on a stretched canvas better. (You can also buy canvas that is on a single board, and not stretched)