This should answer all your questions...
http://www.bdomineau.com/gpage6.html
Just remember that anytime you disc out trash, you run the risk of losing some gold that hits in that area. While I used a lot of disc in the beginning, I basically run in iron mask now and depending on the location, very little of it. I've come to agree with many of those who've posted on the subject that it's good to hear everything and "see" between the trash. Running a pattern in IM and then going all metal can also reveal a good target.
I've tried to develop a disc pattern to reject some tabs only to find that some gold rings I had were also disc'd out. Now you can do some things with the size of the cursors (in learn) used to reject a piece of trash and still be able to accept gold that hits around that general area. The trash item will break up (or be one way only) but the gold item will sound a "little" better as it tries to break through around the disc'd out area of the screen.
Discrimination does come in handy when you're in an area where there is a dense amount of a particular, common piece of trash. You can learn / reject that item and cut down on time wasted digging them. Yes, you still might be missing some gold, but you'll dig more good targets while avoiding, for instance, a pesky type of pulltab or other trash item.
As far as how "different" they sound, from my limited experience, it depends on how irregular the shape of the object is. It also depends on how the object is laying in the ground or sand. I've dug some items that were very lousy sounding but turned out to be chains or small rings. I've found that most trash items are not as "solid" sounding and tend to break off quicker. You can also use pinpointing in your decision to dig or not by checking out the profile or size of the target. If you're looking for a small round object like a ring or a coin and the profile is huge, it's probably trash.
Just a side note, whoever invented the little round foil "tamper proof" tops on sport drinks should be shot. They sound soooo good sometimes
Definitely check out some of Cody's posts on the subject in the Classroom forum.