The categories are defined to be easily manageable and patterned to industry standards.
Social and Affiliation Organizations: Sites sponsored by or that support or offer information about organizations devoted chiefly to socializing or common interests other than philanthropy or professional advancement.
Sites of search engines that provide info exclusively for shopping or comparing prices, however, fall in Shopping and Auction.
Sites that provide advertising graphics or other ad content files, including ad servers (domain name often with 'ad.' , such as ad.yahoo.com).
Also includes institutions such as museums, libraries and historic sites.
Sites that promote historical, cultural heritage of certain area, but not purposely promoting travel.