From what I understand, the discounts changes slightly from year to year. This year, discounts (standard, not holiday, which is something altogether different) include:
1. 50% or 40% off hotel stays, depending on availability and if the CM himself is staying in the room or if it's for friends/family.
2. Discount off all Disney merchandise (in the parks, DD, etc., but not online, as far as I know). I think 20% regularly?
3. There is a cruise discount. I think it's also based on availability but I think it starts at 50% and goes down from there. I don't quite understand how that works.
4. There's also food discounts, depending on the location and time of day. For example, some places give greater discounts at lunch but others only give discounts during dinner Monday through Thursday.
5. And the favorite question, park attendance. Part-time (and full-time, I believe) CMs get themselves in whenever they want. If they're married, the spouse can get in with them. They will also get a main gate pass, which is good for bringing up to three people (the number of people changes; I'm not sure what it's based on) into the parks up to 12 times a year (they all get park hoppers; hooray!). Part-time means working at least two days a week. Once you get the initial training in (several months working two days a week), you can drop down to seasonal, which get all the same benefits but can only get people in up to six times a year, I believe. Oh, and CMs park for free, of course.
I'm sure there' s more I'm not thinking of, but that's the main bits. If you have any other questions, just PM me.