There is no cataloging. Search engines and directories are used to locate documents on the web. These tools act a little like a library catalog, in that they create lists of documents and permit users to search the lists. Most searching is based on keywords and permit the use of Boolean search strategies. Because search engines use keywords they search better for specific words than for concepts.
Directories arrange the list of documents in a hierarchical lists.
None of the search engines or directories list all of the web's documents. So you often need to try more than one to find what you are looking for. Webhelp is an interesting search engine that is human mediated. When looking for some subjects this mediation is very helpful. Searching for information about the use of portfolio assessment in education with most search engines yields articles concerning stock portfolios but with Webhelp I found several useful documents.