BreadCrumbs

A list of links at the top or at the bottom of the page, telling users how they got here, and how they can go back to where they were before.

Hänsel and Gretel followed their breadcrumb trail to get back out of the dark forest again. The trail can literally list the pages visited, or list the pages on the way up from the current page back to the main page (irrespective of how the user got here).

• Orient the user relative to the rest of the website. If the site has hierarchical information architecture, a breadcrumb trail is usually the best way to do this. – Nielsen, Deep Linking is Good Linking.
• The most obvious design lesson from information scent is to ensure that links and category descriptions explicitly describe what users will find at the destination. Faced with several navigation options, it’s best if users can clearly identify the trail to the prey and see that other trails are devoid of anything edible. – Nielsen, Information Foraging: Why Google Makes People Leave Your Site Faster

Kinds

There seem to be two kinds of BreadCrumbs, one of them possibly incorrectly named:

• A trail leading from the first page the user visited, through his browsing history, up to the page he’s on at the moment. This is personal, user-specific and requires some form of saved state, either saved in a cookie or in a server-side session. Only ever shows pages that were already visited and effectively doubles the functionality of the back button. Makes it hard to make CacheablePages.
• A trail leading from the main page of the site, through a hierarchy of sub-pages, up to the currently viewed page. This is page-specific and only makes sense when the site actually has a hierarchy of pages. Instead of a trail, or path, it can be also visualized as a tree or several layers of tabs. Gets tricky or even confusing if a page can be part of several hierarchies.