Do I need an airport transit visa through Switzerland if I have no Schengen visa? Am traveling from Cairo to Moscow via Swiss Air
Assuming that you hold an Egyptian passport (as you did not specify that in the message), you do not need a visa provided you are either: 1) transiting in Geneva airport on the same calendar day or 2) transiting in Zurich and will be there for less than 48 hours.
Also, read these posts on this topic for more information: Is there any online resource that can tell me what visa rules are for passport holders of a specific country? How to quickly find out the entry requirements to country X for a citizen of country Y?
In Schengen area, in most airports (as far as I know, all airports one might transit through), there are distinct Schengen airport zone and international zone.
As a general rule, you cross the border in/out of Schengen only if needed. If your origin and destination are both out of Schengen area, if you stay in the airport international zone, you do not need to cross the border and therefore do not need a visa.
If you want to go to the city for a tourism break in your journey, you should check the country's exact policy regarding transit visa. Several offer such an option. The answer given by @Paul Raftery gives you the details for Switzerland.
If your final destination is in Schengen area, you should cross the border into Schengen at the first airport you fly in.
NOTE: Answer moved from Transit through Stockholm (Schengen) from Beijing to Ukraine? to have a generic Schengen transit answer.