That's a very good question Austin. I don't have the details to hand but I'd imagine it covers things like failing to work as described 'on the box' and any damage caused to the tyre by any failure of the product.
Currently around ten pounds per tyre, so it's usually best applied to a new tyre. Apart from the usefulness of preventing punctures and of course the safety aspect, maintaining correct pressure throughout the life of the tyre is quite likely to increase the life of the tyre as well. The service is carried out by franchised providers such as Continental Tyres, Lewis Land Rover in Swansea, Chuker's of Shrewsbury and others.