What exactly is considered “commercial use?”?

I know there are rules regarding commercial/non-commercial use on many things, but I’m wondering what EXACTLY it means.

For instance…

If you took a picture of someone that you didn’t have sign a release for commercial usage, and you put it on a site such as Flickr, it does not directly bring you money. But if you were to have some photos that ARE okay for “commercial use” up for sale, indirectly that photograph could possibly bring you profits. Is it okay to do that?
And would it be okay to show it on a site where you are making money from ads (such as those google ads), even though you are not selling the photograph itself?

If you use a non-commercial photograph on a non-profit website that accepts donations solely to cover web hosting fees, is that using it “commercially?”

Would it be okay to use a non-commercial photograph on a non-profit website that accepts donations to cover web hosting fees and other charity work?

A web site has graphics to use in other digital artwork, provided you do not use it commercially and do not redistribute it. You use it, don’t make any money off of it, but you are making graphics for others on a small scale. Are you breaking the terms of use?

I guess basically I’m looking for the whole outline on what EXACTLY it all means, and in english rather than regal legal talk.

Despite what I discover, I plan on doing everything legally. Even if I can “get away with it”, I want to set a good example and give a good reputation for photographers/charity runners/digital graphic designers. It’s their work, and I want to respect that; please don’t just answer with a “who cares” or “just do it anyways”, because I won’t.

Thank you!!!