The best introduction to this subject is at the Joy homepage.

In short, concatenative languages behave in a way which looks imperative (like C or Perl), but can be reasoned about in a functional manner (like ML or Haskell).

These languages are only beginning to be studied, although Forth had a heyday in the 80s, and Postscript is notable for being the most commonly metaprogrammed language in existance (a trait which I believe is not a coincidence).