First off, if you don't know who MacPhisto is, how did you get here?? During the Zoo TV/Zooropa tour, MacPhisto was Bono's character for the last part of the show. According to the FAQ, he was used during the European tour because they thought he would be easier for them to understand than the evangelist character of Mirrorball Man. MacPhisto is obviously the devil and if not, then an evil character (big suprise). Bono comes onstage in a gold sparkled suit with matching platform shoes, pasty face makeup with dark eyes and red horns. Bono has said the character is a cross between Mephistopheles (the devil from Faust, go get the book), Gavin Friday and about 500 hundred other people. 
And um....thats about it. 

The Lair