Basically, the certificates differ with regard to the security level that is guaranteed by the varying intensity of validation processes undertaken by the certification authority (CA).
There are three types of TLS/SSL certificates:
domain validation certificates (DV)
organization validated certificates (OV)
certificates with extended validation(EV)
The domain validation is carried out via a TXT in the domain zone, a file auth entry or through confirmation of the email sent by the CA.
With organization validation, the domain is also validated as with DV certificates. Additionally, the CA also checks the organization applying for the certificate. This is carried out by using public resources such as the commercial register. Further checks are undertaken by calling the organization. To do this, relevant telephone numbers are found by checking public registries.
With extended validation, the domain and organization are validated according to the processes described above. Over and above this, the specified administrative contact is also verified through an email authentication process.