Warhammer is a table-top miniatures game created by Games Workshop. It is split into three different games:
Warhammer Fantasy - Orcs, Goblins, Knights, etc
Warhamer 40,000 (40k) - Aliens, humans, guns, etc
Lord of the Rings - Based on the books by JRR Tolkien.
The player builds an army of whatever he wants, glues them together, and paints them, then plays games with them.
The game itself is very simple to understand, but also very complex. Games can last hours and are very fun to play.
Painting an army can take a long time or a short time, depending how much effort you put into it. Although some people finding painting a burden, to me painting is half the fun of the hobby.
Warhammer is somewhat overpriced. However it is not just the plastic/metal soldier you pay for, but also the hours of enjoyment you get from it by painting and playing with it. This makes it worthwhile.
Warhammer is a great hobby!