I was going to start this thread with a bunch of excerpts from the Wikipedia entry on concept albums. But it's too much bloody work. Suffice it to say that the Wikipedia entry on concept albums tells you just about everything you want to know about 'em.
For a long time I've had my own little sacred triumvirate of concept albums:
The Who - Quadrophenia
ELP - Brain Salad Surgery
Jethro Tull - Thick As a Brick
Part of the reason I wanted to talk about concept albums is that I'm still obsessed with this album by The Waterboys called 'An Appointment With Mr. Yeats' which came out a couple of years ago. I put up a thread about this album a while back. I was listening to it again last week and I'm convinced that it's one of the best concept albums yet made. If it had come out in the Seventies I think it would have been recognized as such. In a way, it came out forty years too late.
I'll talk some more about that, and about many other fascinating things about concept albums. Won't you join me? :-)