I've done the Christmas tour a couple of times. They give you headsets so you can hear them talk, and they play holiday music for you while you walk around to get you in the spirit. It was funny last year when our tour guide wanted us to sing along to the music on our headphones, and people around us were staring at us because we were all singing, but they didn't hear any music to sing with.
They tell you about the various decorations throughout the areas of the park, and you get a free piece of fudge (my favorite part!) along the way. You go on Haunted Mansion Holiday, and then take the train over to Small World Holiday. After the tour is over, you get special seating for the Christmas parade at the area near Small World, and they give you some hot chocolate and a cookie, plus a pin to remember your tour. I obviously enjoy this tour since I've done it several times. In the past you could get a discount with your annual pass, or a Disney Visa card during certain dates.