Things were essentially VW Beetles built for a more rugged life--and they looked the part. All the rounded, 1930's styling was banished and replaced by flat reinforced panels. It's almost like someone decided to build a VW out of lumber in their back yard.
They were a continuation and improvement over the Kübelwagen, which was used by the German military during World War II.
Nowadays Things are about as rare as seeing a silver quarter, but they always bring out a smile.
See DasTank.com, the Thing fan website, for much more information.