What happens when I close/abort a WCF channel/proxy?

I'm trying to get a better understanding of what's going on when I use a WCF proxy. I'm having trouble understanding what happens when I close (or don't close) a proxy.

• What's going on when I call Close() or Abort() on a WCF proxy? What's the difference?
• How does it differ between types of bindings (like, a sessionless BasicHttpBinding vs. something sessionful)?
• Why can Close() throw in certain situations and why can it be a blocking operation?