Think of a proper list like a degenerate tree with all the values on the left and structure on the right. A dotted list puts the last value on the right, where nil would usually be.

Dotted lists are pretty rare. They mostly show up in association lists, where key/value pairs are stored as

  ((name . dave) (type . user))

and adding a pair to the front of the list shadows any other pairs with the same key.