Wampum were basically shell beads typically made from quahogs.  This wiki page can give you some idea although there is some debate as to exactly how the Native Americans used the individual "wampum" beads.
http://en.wikipedia.org/wiki/Wampum

And here is the brief history we wrote up on our site as to why we used the term wampum:  http://wampum.org/history-of-wampum/

I was raised in RI on the Narragansett Bay, where wampum was typically made through a very difficult manual process.  When the colonists realized the Native Americans valued these beads they started mass producing them in factories which drove the value down and made them basically worthless.  The term is not used much today but when I was a child my grandfathers used it often to refer to something of no value.    My great grandfather ^11 Roger Williams, is actually the one who anglicized the term and put in the first translated dictionary.