Swiss German is a German dialect, and from what I know it is pretty different from standard German. Where else is German spoken? Well, anywhere, really... but it is spoken as the dominant language in Germany, Austria, Switzerland and Liechtenstein, and has official status in Luxembourg, Belgium and Italy.

Depends on what you mean by "major". It sounds very differently, though it looks very similarly when it is written. There's a bunch of words that are different. It's not that much different from a strong German dialect (political correctness aside). Some native German speakers won't understand it at all, some native speakers can understand it after some time when they get used to it. A Bavarian trying to understand Friesisch would have the same problems. For a foreign speaker it will be really hard to understand if he or she has learned standard High German pronounciation. Fortunately, most Swiss can "switch" to High German, or at least an approximation. Think of a strong English dialect like Scottish, it's about like this. Someone who has only learned the standard English pronounciation won't be able to understand Scottish at all.

German is also widely spread in Namibia. And also many mennonites in Belize speak and understand German. They learn it in school and the bibles in their churches are in German, too.

ich verstehe kein deutsch...at the moment

Ich kann das Schweizer-Deutsch nicht verstehen und es hört sich in meinen Ohren schrecklich an. Es gibt viele Lautverschiebungen. Am besten kann man in Norddeutschland Deutsch lernen, zwischen Hannover und Hamburg!