[5.3] What's the difference between a sprite and a channel?

Not much. A sprite is a channel with something in it. A channel with nothing in it isn't anything at all -- it won't appear onstage and you can't alter it with Lingo.

It's important to note that some sprite properties actually affect the whole channel, and remain in effect even if you jump to some other part of the movie where some other sprite is in that channel. (The puppet and visible properties are the important examples of this.)