Wish You Were Here was the album after Darkside.
It has to deal with the abscence of things.
Such as friendliness in a handshake (Machines shaking hands, "Welcome TO THe Machine" )
The 9 part song "Shine On You Crazy Diamond" is about Syd Barrett, one of the founding members who was forced out due to the fact he was losing his grip.
The title track was sung by David Gilmour. It is one of the greatest Floyd songs ever.
"Have A Cigar" was sung by Roy Harper, not a member of Floyd. He was invited because Waters couldn't seem to get it perfect.