Obviously the timing belt needs to be replaced. Then they could change any followers or guides which mechanically get worn out. I would always recommend getting the water pump changed if it is driven off of the belt, or located in the same vicinity as the belt. The reason for this is, if you have to remove everything to replace the pump (ie: timing cover, belt, followers, re-time the camshaft(s), etc) to do the work, if the pump were to go out at another time, you'll end up spending for a second round of labor to get it replaced. Labor is the biggest part of the bill on doing the timing belt. In most cases, it is literally five to ten more minutes of work to replace the pump. Most timing belt kits come with a water pump. The cost of most of them are very incidental to the overall cost of getting the timing belt replace. For peace of mind sake, just do the pump at the same time. You could view changing the water pump as pro-active maintenance as you're doing it before it fails.