What is piracy all about?

I mean, is it not paying for copyrighted music, or is it SELLING copyrighted music that you didn't pay for. I mainly want to know because there are a couple songs on YouTube that I reeally want to get, but I'd have to get them with my dad's iTunes, and he doesn't have the money for it right now. So I was going to download the (crappy version of the) mp3 from the music video until we had the money, and I don't want to break the law, so is this a bad idea, or an okay one?