We've already seen an amazing display of coding skills over the Breadcrumb trail system, now I'd like to share some thoughts about the reasons for using it.
Other Breadcrumb articles at evolt
The term breadcrumb immediately evocates the tale of Hansel and Gretel, where they dropped little bread crumbs while they were entering the forest in order to find their way back, a bright strategy.
Some websites have turned into deep dark forests of information, where the users can easily get lost without the use of breadcrumbs, therefore some clever architects developed the web analogy to the tale's breadcrumb trail.
Simplifying the picture, I see it this way: main navigation should tell me "where I can go", and the breadcrumb trail is an aid telling me "where I am" within the site, it's not a rigorous homologue of Hansel and Gretel�s trails, telling me exactly the path I've followed to get there (there are always search tools and sitemaps) but an indicator to "you are here", more like the maps in the shopping mall, giving the user a sense of location within the general hierarchical structure of the site, plus the possibility (through hypertext) to jump up multiple levels, this is useful when one wants to travel through content chunks in a "particular to general" direction.
Whether if you use breadcrumb trail in your site, where to put it within the page and how it will look, depends on each project: the content, the audience, and ideally, some sessions of user testing.