The main goal of Web Authentication is to certify the identity of a legal entity or legal person (such as an organisation, a company, an institution, a government agency, etc.) that is behind the services offered by the website or web service it protects.
Web authentication is used to reduce the risk of phishing, i.e., the risk that genuine clients get attracted or redirected to fake websites that will steal their data and access to the real service.
Web authentication can be achieved with SSL certificates. Those certificates are also used to enable the encryption of transmissions between server and client so that data cannot be tampered while flowing in the transmission channel.
There are many types of website authentication certificates, which depend on similar qualities of cryptographic processes, but they are also different in terms of the procedures used to establish the true identity of the organisations or individuals that purchase them. Their value is only as good as the trust in the verification of the identity.