A B2B directory is fundamentally different from a search engine, even though it also presents a list of links to a certain topic. Search engines look for sites on the whole B2B based on the keywords one has entered, whereas a B2B directory has a set amount of links, usually entered by the owners of the corresponding site, which are sorted according to topic, usually not more than one or two per site, and always referencing the whole site, not simply subpages of it. Directories may vary in size and scope, some only dealing with a small amount of topics or being specific to a small geographic region, where as others might be extremely large. Examples o the latter are Yahoo! Directory and the Open Directory Project (ODP), which is huge and open for search engines and other directories to use. There are several different types of directories and ways they treat listing. Some offer their service free of charge, some demand a one time or recurring fee, some even let you bid or pay for the best positions within the directory.
Some directories, especially the free ones, put restrictions on their listings for example they could make it impossible for search engines to follow the links behind the listing. In addition, some might require a link back to the directory on every page they add to their listing. B2B directories basically work like a telephone book for the internet, all you need to know is the starting address for the directory and what you want to find.