N idiot’s guide to software licensing somewhere

Basically, my knowledge on the issue is zilch other than the fact that open-source and closed-source exists.

I'm a web developer (not a designer in the slightest), so I look online for things like icons. I've always been a big fan of these icons, which have a Creative Commons Attribution 2.5 License. As far as I can see, this license says 'do whatever you want with them as long as you have a link back to me somewhere'. Is that assumption correct?

Just today I found a new icon set, with a much more confusing license (found here), and to be quite honest I have no idea if I'm allowed to use them or not.

• At the moment I want to just use them for toy stuff that might never see the light of day, but then my source code is stored on Github, is it legal to store the icons there where they're publicly accessible?

• If I put them on my personal website that might have ads on it to make me five cents every now and then, is that legal?

• If I use them on a site that offers a free service to users, is that legal?

• If that site then starts making money (via things like paid subscriptions) or gets bought out by someone (highly unlikely but one day possible) is that legal?

Is there some noob guide out there that explains all this stuff, because I would hate to start using this sort of stuff now only to have to change it all later. Even if I buy the icons, there's still licensing issues that I don't understand! 🙁

And this sort of stuff keeps popping up more and more often…