As an American considering a post-doc in Australia, how does superannuation affect me? I have been offered a temporary (3 years) post doc in Australia with a salary and superannuation. 

As an American, 


Can I access this money? Is there a penalty? 
Can I transfer it to a tax-advantaged American account? 
Is it held in an investment vehicle? Can I choose it? 
If I do not retire in Australia, can I still let it sit in an Australian investment vehicle until I am retirement age and then withdraw from it?
The comment above about non-specificity to academia is mostly correct, but there are special considerations for academics.  Australian academics (including postdocs) typically receive their superannuation from Unisuper.

All Australian academics need to know that the rate of superannuation contributions varies and that there are defined benefit and defined contribution systems, depending on your contract you may not have the defined benefit option.  Most Australians do not have defined benefit.

If you are a nonresident you can (and are incentivised to) withdraw your superannuation when you leave Australia, but there is a big tax.  You may be able to obtain residency in order to avoid this tax, but it is very complicated.

If you are an American, you need to study up on Passive Foreign Investment Corporations.  In short, this is a tax the American government imposes on superannuation.  There is no way to calculate how much tax is owed.  The IRS says it will take you 40 hours/year to complete the forms.  Despite this idiocy, it is worthwhile to work in Australia.

I am not a lawyer.  Talk to someone who specializes in international taxation and/or investments.