Sure, you've seen The Lord of the Rings trilogy in full-length, extended versions, all in a row, like seven times by now, but do you know the exact intentions of the "One Ring" or do you just get caught up in what's happening to the characters? I, for one, never really gave thought to exactly what the ring does. I know it makes you powerful, and I know that everyone wants it and that destroying it killed Sauron, but why?
Watch this fun, well-animated and thorough video with a 4-minute explanation of the ring that'll make you sound like a huge, cool nerd at your next movie outing with your friends who don't spend all day on the Internet at work.
And after about a minute and a half of nerd talk, if you feel like this...
Here's a TL;DR: The one ring was forged by Sauron, along with a bunch of other rings, so that he could control people's minds and rule the world. He did this with the help of a powerful elf, but I'll talk about that in a few sentences. He tried giving dwarves some rings, but all they want is riches, and they sure got a lot of them, so that attracted dragons, who ate the dwarves. He tried humans, but they all turned into bloodthirsty wraiths. And Elves were too powerful to fall under his control.
There was war and the elves won and Sauron/One Ring were gone.
What does the ring do? The ring always wants to go back to Sauron, and its primary use is mind control, so it tricks anyone into bringing it to Sauron by convincing them they can use it for their own purposes. They can't, that's a lie.
It also makes people invisible, which is horrible because it attracts the wraiths, which will kill you, while you're invisible, so it's really useless to anyone but Sauron. He's so connected to The One Ring that when it's destroyed, so is he, along with all the other rings.
BONUS: Why are the elves so sad? The rings of power were forged by Sauron and that powerful elf initially to preserve magic in the world and protect their kingdoms. Ever notice how the elven kingdom is so badass? That's because they're magical beings and it's all protected by magic, which their rings were helping protect. With the rings going away/The One Ring being destroyed, it spells the end of the time of magic in the world, and the beginning of the time of man... according to this video.
There ya go, saved you three whole minutes!