Property maps

The documentation for boost::property_map is awful, so would someone please explain to me what this implementation does and how it differs from std::map<> and std::multi_map<> ? I know this post sounds irascible and I am not trying to dump on Mr. Siek but really I do not understand what this idea is or why one should use it over and above what already exists in the C++ standard library. I only wish that Boost implementors would stop explaining their ideas through examples and start explaining them in logical ways in which a knowledge of the English language would suffice.