A long time ago, someone invented Java. It's all about security, so is ideal for the Internet (actually, it hasn't done quite what was intended for it, but it's doing pretty well). Anyway, then someone invented JavaScript. The only thing it has in common is that it looks a bit like Java, beyond that, it's nothing like it.
To cut a (very long) story short, JavaScript is used to do all sorts of things on web pages. Most usefully, it can check what you've typed into a form and tell you if you missed something (without having to wait for a whole page submission). So it's great, right?
Wrong. Enter Microsoft. As always, Microsoft couldn't leave well alone, so when they implemented JavaScript for Internet Explorer, they buggered it up. That means that when people make websites and only test them in Internet Explorer, they don't work in any of the standards based browsers (meaning every other browser in the world).