Hey there Woodchuck, I'm Blake from Home Depot's online community. This is a great question, hold on tight because its going to get nerdy!
Essentially when a freezer is cooling, condensation will accumulate on the cooling coils. This condensation will freeze and build up. After 6-8 hours a defrost timer shuts off the fan, and turns on a heating coil that melts the ice off the cold heat exchanging coils. The melted ice then runs down a drain line into a pan that sits near the hot heat exchanger on the bottom of the freezer. When the cooling cycle starts back up the hot coils of the heat exchanger will cause the water to evaporate into the room. Having the water close to the hot coils actually helps the fridge run more efficiently (think water cooled engine VS air cooled engine).
Because the freezer evaporates all condensation there is no drain line needed. I hope this helps Woodchuck. If you have any other questions we would love to see you over on the how-to community
http://Community.HomeDepot.com-Blake