There are actually two types of "yellow tea". This is to my understanding, so don't take it as 100% gospel, but I haven't found any conflicting information yet. I read a blog entry, some time ago, of someone that went to China and visited some farms to find out just what yellow tea is about. I can't find the article now, but will try to do so and edit this post to include the link if I find it.
There's "yellow tea" and "Yellow Tea".
In the old days, the emperor would collect taxes in the form of either money or goods. The emperor would only take a relatively small amount of tea per year for consumption by the royal court (or whatever it's called). So the farmers would try to produce the best of the best so that the emperor would choose their tea to take as payment for their taxes. Since yellow was the official color of the emperor (I believe that ONLY the emperor could wear yellow), this kind of tea became known as "Yellow Tea". This is also where the "Imperial" grading term came from, but it seems to me that term gets abused a bit these days.
Then there's "yellow tea", which is a distinct class of tea. As Mary noted, it's like green tea, but with a much more complex drying process. Unfortunately there are, apparently, not many people left that know the secret of making yellow tea. There are also a lot of green teas sold as yellow tea, because yellow tea costs a lot more due to the more extensive and skilled processing. AFAIK (both from my encounters and reading specifically) the only yellow teas out there are Jun Shan Yin Zhen and Huang Ya. Real Jun Shan Yin Zhen gets copied quite a bit, and commands fairly high prices. How else can you charge something like $300/lb for green or white tea? I've not seen the real stuff sold outside of teaspring.com.
I wrote a blog entry on Jun Shan Yin Zhen a while ago. If anyone wants to see it, you can find it HERE
If anyone gets Jun Shan Yin Zhen, I would strongly recommend getting a tasting set for it. This is the only thing I've been able to use that will "reliably" bring out the more interesting characteristics. Not even a gaiwan has been able to do the same.
My local tea shop also seems to call some of the more amber oolongs 'yellow tea'. I'm not sure exactly what the story is there, maybe it's a Cantonese thing, or maybe it's just the grading, I really don't know and I haven't asked because of the language barrier. One of them he calls "Yunnan Yellow Tea", which tastes like a particularly fragrant dancong but has very long (up to 3 inches), straight, striped leaves (not like dancong that I've seen before, and not like the normal dancong which he offers as 'phoenix dancong'), and the other is Da Hong Pao.