From Seo Wiki - Search Engine Optimization and Programming Languages
Unlike later search engines, like Aliweb, which attempt to index the web by crawling over the accessible content of web sites, W3 Catalog exploited the fact that many high-quality, manually maintained lists of web resources were already available. W3 Catalog simply mirrored these pages, reformatted the contents into individual entries, and provided a Perl-based front-end to enable dynamic querying. 
W3 Catalog was retired on December 8, 1996.
- ↑ Oscar Nierstrasz (2 September 1993). "Searchable Catalog of WWW Resources (experimental)" (html). http://groups.google.com/group/comp.infosystems.www/browse_thread/thread/2176526a36dc8bd3/2718fd17812937ac?hl=en&lnk=gst&q=Oscar+Nierstrasz#2718fd17812937ac.
- ↑ 2.0 2.1 "W3 Catalog History" (html). http://scg.unibe.ch/archive/software/w3catalog/.
- ↑ Thomas R. Gruber, Sunil Vemuri and James Rice (December 1995). "Virtual documents that explain How Things Work: Dynamically generated question-answering documents" (html). Knowledge Systems Laboratory, Stanford University. http://www-ksl.stanford.edu/people/gruber/virtual-documents-htw/.