Can anyone explain what curves does, and I also have questions about best processing methods.

Tutorials:

http://www.ronbigelow.com/articles/curves-1/curves-1.htm

http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

Basic adjustments:

Fix horizon (optional), crop image (optional), retouch adjustment (if necessary, remove dust), levels adjustment (or curves; start with AUTO), highlights/shadows adjustment (optional), resize/export to web. You may need some sharpening in there also.