Corporate sites differ from other sites viz. electronic commerce, portal. In that they provide information to the public about the company rather than transacting business or providing other services. The phrase is a term of art referring to the purpose of the site rather than its design or specific features, or the nature, market sector, or business structure of the site operator.
Nearly every company that interacts with the public has a corporate site or else integrates the same features into its other websites.
Large companies typically maintain a single umbrella corporate site for all of their various brands and subsidiaries.