"The Green Screen Show" stars seven to eight improv performers, as well as Drew Carey (the host of the show), who act out random scenes behind a part of the room where it's completely green, hence the "Green Screen". And, as Drew would say, thanks to the magic of television, cameras can project images on the green screen without the actors seeing them.
The main reason as to why the "Green Screen Show" is different from "Whose Line is it Anyway" is that the "Green Screen Show" features a wider range of improvisational actors and actresses. However, we have actors that starred in "Whose Line" to come to the "Green Screen Show" like Greg Proops, Kathy Greenwood, and Colin Mochrie. What really dissappointed me about the cast of characters is that Ryan Stiles does not appear as a regular on the show. Without him, I cannot fully see the humour behind the "Green Screen Show". I'll still catch him in the commercials, though.
The "Green Screen Show" crew definitely keeps the show well organized, as their visuals, looking complex and all, sets the scene smooth enough for me to laugh at (not in a "haha, this sucks" way). I also like the "props" they place on the actors' body, and how it follows the actors' movements. It's really nice to see such professionals, if not masters, at work by looking at what they've done for the show.
Drew Carey is definitely the King of Improv, hosting such shows as this one and "Whose Line is it Anyway". While the "Green Screen Show" is not, I believe, his best feat, I still watch it on a somewhat regular basis, trying to enjoy what it does naturally, although it does fail me time to time, but it's a great show nonetheless.