First opportunity for private loan from an aquaintance... how to?

[Matthew Defore]

I have my first opportunity to take a private money loan from an acquaintance.  This will be a first for the potential lender as well.  I am still kicking around the idea, and I want to learn more about the concept first.  My first question is, how formal is a private money loan under these circumstances?  Is it as simple as receiving money from the acquaintance and repaying it according to the terms that we establish?  Can this be an agreement written between us, or is there any regulation involved?