A multi-entry Schengen visa is usually valid for 6 months, so you can make numerous journeys/visits to Schengen countries.
However, there is a grey area.... basically the terms of the visa say that your point of entry must be the country that approved the visa.
If you intend to visit several Schengen countries, you must apply for a visa at the Embassy or Consulate of the country which is your main destination.
If you intend to visit several Schengen countries but do not have a main destination, you should apply for a visa at the Embassy or Consulate of the country which is your first point of entry.
so, effectively, if Austria granted your Schengen visa, then you should enter the Schengen area via Austria.
How diligent they are enforcing this (especially after that initial first trip) is anyone's guess ... and I'd love to know if anyone has "experience" of this.
A single entry does what it says on the tin... you enter the country once, and after you've departed, it expires.
You then need to apply for a new Schengen visa if you want to travel to that country again.
