Thanks Jila. So "Sunnah" means simply recommended and not compulsory?
'Sunnah' is short for 'Sunnat Al-Rasool', which literally means 'the path of the prophet'. A sunnah is something that's not ordered by God but nevertheless the prophet did for extra credit. And since Muslims believe that they should follow everything the prophet did, it's considered good to do sunnah stuff.
But there are levels of sunnah. There's 'sunnah mu'akkadah', which is an emphasized sunnah, and it's considered almost on par with something compulsory. Then there's 'sunnah ghair mu'akkadah', which is a non-emphasized sunnah, and is something you'd get credit for if you do but don't lose anything if you don't do.
As a side note, the term 'Sunni' comes from the same word.