Basically elite special forces soldiers - among the best in the world. The term 'Special Air Service' was adopted during the Second World War partly to conceal what they really did - it made them look like they had something to do with planes when they didn't - and the reputation they forged meant the name became one to be proud of, and so it is retained.
God bless Australia!!
The best ally of the USA-even better than the even the British.