Google Glass is a head-mounted display in the shape of a
pair of glasses. It is activated by voice and is supposed to stream the
internet directly into your vision, so you experience it more directly. You can
get an idea here
of the kinds of things you will be able to do with it - for example, if you're
watching something you can say 'take a picture' and it will take one.

At the moment you can't get one, unless you are a US resident and able to enter
a competition by 27 Feb (you can find details by following the above link and
clicking on 'how to get one'. However, even if you win you have to pay for your
Glass.

Part of the competition is to find creative ways of using it, so to your
question: Is it useful? The answers are still evolving. It certainly has a lot
of potential, but many people are worried about privacy - for example, somebody
wearing a Glass could video the person talking to them, without the other
person knowing it.