Both the above, but if you're looking to really save money, get the bus. Out of the terminal and the bus stop is on your right. You can buy a (3-day?) tourist pass from the tourist booth just by the exit.
We used buses and trams to get around with no problems. The train to Oświęcim is slowish but a very interesting trip through rural Poland. We got a coach back from outside the main museum.
Er, edited to add info is two years old!