Austrian German is actually a variety of Standard German that is spoken by more than 98% of the population of Austria.
An excellent way to understand the differences between Standard German and Austrian German is to compare British English and American English. The languages are broadly similar with a few changes in grammar, vocabulary and expressions. Pronunciation remains essentially the same.
Austrian German took its roots from the late 18th century when the Austrian government established administrative standards in languages. As the years progressed, German in Austria became influenced by the Bavarian and Alemannic dialects of Germany to give the Austrian version a sense of individuality.