The key difference between HD DVD and Blu-ray is the type of physical media they use -- for everything else, these formats are essentially the same. They mostly use the same video codec, the same audio codec and, on the whole, the interactivity is comparable, they're just run by different software -- Java on Blu-ray and Windows CE on HD DVD.
So, why is there aat all? Sadly, it's got everything to do with money and absolutely nothing to do with giving the consumer a good deal.
When a company creates a technology, it makes money from the royalties earned from licensing it to other companies. This money, in many cases, can outstrip the cash they make from actually producing and selling the equipment itself. JVC made a boatload of cash from VHS and both Sony and Philips made a good-sized wodge from CD patents.
It's this that drives companies to create competing formats. These companies try to convince us that it's their format that's better so we'll all flock to the shops to buy products that use their format. That's the problem with HD DVD and Blu-ray -- Toshiba wants us all buying HD DVD; Sony wants us all buying Blu-ray.
But it could all be solved very simply, and here's how: a law is passed that states all films released on Blu-ray must be available on HD DVD and vice versa. Oh, and the same law will apply to SACD and DVD-A. We then back this up with a second law, which states if it isn't available on both, then the consumer can legally download it off the Internet. Mark my words, the format war will be over in the blink of an eye as opposing camps magically set aside their differences.