Yeah, they make several different ones, but they're all a little UV thing that you put in the water, and it irradiates viruses and bacteria. Doesn't help with things that aren't alive (so if you were in an area with a history of mining that had traces of arsenic in the water, like Monte Cristo off the Mountain Loop Highway it wouldn't help you) but for giardia and just about anything else you'd normally worry about, it's a very quick and easy way to protect yourself. It's fast enough that the water is still cold after it's been purified.

Here's a picture of mine in action, from a backpacking trip on Mount Baker.