What's the difference between an expander and a noise gate and are there other uses of an expander besides reducing noise

Fundamentally no difference. You can be creative with expanders. I had some live drum layers under a sequence and used an expander to emphasize the fills, so it rode through the beat under the surface subtly and not adding clutter, then jumping up for the builds and silly crashes and tom rolls. They are good as fake sidechains! Stick an expander on a drum buss and, if you set it right, you can have the beat rolling up and down between the BDs and SDs. Sometimes this works great just on a ride/hihat track, as it will pull up the accents even more and bounce the rest around. Reaper lets you add white noise to the signal when you monitor. That helps a lot to hear how it's operating. Otherwise you can add your temporary filler sound if it would help, for any dynamics. I'd love to hear some other ideas to do with expanders. I bet there's some awesome tricks.

Expander controls : ratio , threshold attack release can someone explaind these

And what's the difference between upward and downward expansion