What is SSL?
Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL) are internet standard protocols used to protect sensitive information that end users send to web servers, such as passwords, credit card numbers, email content and private messaging.
How does Certificate Authority (CA) work?
CA is a trusted third party that issues digital certificates, and validates web server information including domain and public key.
1. The client’s request for an SSL certificate generates a public as well as private key, and sends the public key to the CA.
2. There on, the CA verifies the client’s legal documents.
3. CA reviews the integrity of the client through several online resources.
4. CA validates that the client is holding the private key.
5. CA examines the client’s public key.
6. After approval, once the CA receives payment, it signs the certificate and public key.

Expired certificates may cause merchant stores to lose potential business
If a customer encounters an expired certificate while shopping, 57% of enterprises say customers will continue the transaction anyway or contact the site for help, but 63% of consumer responses say they will abandon the site and only 27% of customers contact support for help.

Intermittent SSL or Always On SSL?

Intermittent SSL only protects the selected page, which means there is still a possible risk of attack with sidejacking methods, while Always On SSL provides full protection of internet data. However, some merchants may worry that it will slow web server response, and not be friendly for SEO. In fact, with modern servers and the latest browsing technologies, the delay is a minor issue. Watch Matt Cutts comments on websites going 100% HTTPS