maps store key/value pairs. For example:
multimaps are similar, but they allow multiple values for a particular key (or duplicate keys, depending on how you look at it). So you could have:
foo: bar, bar2
biz: baz, baz2, baz3, bleh
There's really no advantage of one over the other -- it's just a matter of whether your use case involves multiple values for a single key or not.
Scott J. Kleper
Author, "Professional C++"