A website that generates revenue or cash flow of any type that isn't under a non profit organizations filed with that state. So if your selling a product, selling advertisement, selling a service, your site is commercial. Whether you plan to do so to make a profit or just to pay your hosting bill. If you accept revenue for your site their is a chance even if you don't try to that, that revenue will generate a profit. Also I saw statements of people wanting to intergarte content from amazon.com, yahoo, monster.com, hotjobs.com, etc. People THINK those are major corporate entities. Their not going to let you use their content for FREE. Yahoo just had a legal battle with sex.com over this type of nature. These companies do offer affilate programs but you have to abide by their terms and service. We are getting to lax wanting everything free.
Again I state just because postnuke is free not every module, block, and plug in is also free.
If you use something that you thought was free but actually it wasn't and you get caught and prosecuted. You will have to pay penaties. If you run a commercial site just be careful and look at every module and plugin especially if they pull or use sources from other websites. You have been warned!