I'm not going to go into a super deep history of copyright here. What I plan to do is outline the general idea behind copyright and how it's shifted over time. If you want an in depth discussion on this I suggest reading it from Lawrence Lessig's book Free Culture which the link to a direct download isn't working right now. I can email it to you if you'd like, I believe.
Basically, the idea of copyright came about around the same time as patent protection. The idea, which is rooted in the same theories as patent protection, is to give just reward to people who created creative works, without fear of someone stealing it, and selling it for their own gain. This worked extremely well in a time when to listen to a great piece of music you had to listen to it live. This is where the public performance aspect came into affect. Of course, books were much more difficult to copy en mass as it's really expensive to print a run of books. Copyright was originally much shorter in length of time than it is currently. At the longest it was until the creator of the work died. Now, as we know, copyright can persist significantly longer. We have more interaction because the material is more accessible. In the early 1900's there was a big congressional hearing about sheet music. If sheet music should be allowed at all. Today, I think our version of sheet music is the video or musical remix, al a GirlTalk. I feel like I'm really pimping Lessig today, but he also wrote a book specifically about this topic called simply enough Remix. In this book he interviews GirlTalk about this very topic.
So, how does this shift in copyright really impact us? Well, as we can tell from the O'Dwyer case, RIAA, MPAA, and their equivalents in other countries, we interact with copyright on a daily basis. Not enough of us have a clear understanding of how we're actually interacting with it. It's conceivable that the video commentary out on Youtube for video games is technically copyright infringement. However, it would be stupid for video game publishers to go after the Youtubers, like my friends at KBMod, because they are effectively giving free advertisement for these games.
I suggest people that are active in posting videos online on Youtube become aware of the copyright issues that you may be interacting with. Sadly, ignorance is not a defense. The books I've mentioned above are easy to read. Lessig writes in a very clear way that non lawyers can understand easily.