SSL (Secure Sockets Layer) certificate is a validation and encryption tool, part of the HTTPS protocol, which secures and encrypts data going back and forth between the server and the client browser. The reason why SSL is used is to keep sensitive information sent across the Internet encrypted so that only the intended recipient can access it.
Single-domain SSL — this certificate has only one SAN (Subject Alternative Name) field and it contains a reference to a single website.
Wildcard (*) SSL — when that one SAN field contains an asterisk in the website name (e.g. ‘*.hexometer.com’) then it’s a wildcard certificate. This is a variety of single-domain SSL but for an unlimited number of subdomains (a domain that is a part of another domain).
Multi-domain SSL — when the certificate has many SAN fields. Multi-domain certificates sometimes have more than 100 SAN fields, and some of these fields may contain wildcards, creating a hybrid “multi-domain wildcard” certificate. This type of certificate is often provided free of charge by CDN (Content Delivery Networks) services. The main advantage of multi-domain SSL — they are mostly free of charge.
To diagnose whether the site has a valid SSL Certificate installed or not click here https://hexometer.com/ssl-certificate-checker