It all seems very clear and obvious in the documentation. The introduction has an excellent and simple introduction. There are also documented motivations and examples in the tutorials.

Please see:
http://www.boost.org/doc/libs/1_39_0/doc/html/function.html
 

If you have any specific questions then please post with a little more information. I'm unable to guess what information you might need currently.

Regards,
Neil Groves