I'm not a bagpipe afficionado. How do you tell the difference?
Well, there are hundreds of different pipes, but if you mean between the more well known Highland Pipes and Uilleann pipes, you can see the way the air is blown into the bag (blowpipe = highland, bellows = uilleann) and of course how the chanter and drones sound. Highland pipes also only reach 9 notes, uilleann has more I believe. I love both, but if you want a more mellow sound, uilleann is your pick.
In this video you can compare the two different sounds:
https://www.youtube.com/watch?v=fNhWxH_NUh8EDIT: You can't hear the Great Highland drones on that video. I think he might have plugged it due to the tuning. The drones you hear are the Uilleann pipes only, I think. The chanter is what plays the melody, and you can clearly hear the difference here.