What is a Single Root SSL Certificate?

When connecting to a webserver over SSL, the visitor's browser decides whether or not to trust the website's SSL certificate based on which Certification Authority has issued the actual SSL certificate. To determine this, the browser looks at its list of trusted issuing authorities - represented by a collection of Trusted Root CA certificates added into the browser by the browser vendor (such as Microsoft and Netscape).

Most SSL certificates are issued by CAs who own and use their own Trusted Root CA certificates, such as those issued by GeoTrust and FreeSSL.com. As GeoTrust and FreeSSL.com is known to browser vendors as a trusted issuing authority, its Trusted Root CA certificate has already been added to all popular browsers, and hence is already trusted. These SSL certificates are known as "single root" SSL certificates. FreeSSL.com, a subsidiary of GeoTrust, also owns the UTN root used to issue FreeSSL certificates.

Some Certification Authorities, like Comodo, do not have a Trusted Root CA certificate present in browsers, therefore they need a "chained root" in order for their certificates to be trusted - essentially a CA with a Trusted Root CA certificate issues a "chained" certificate which "inherits" the browser recognition of the Trusted Root CA. These SSL certificates are known as "chained root" SSL certificates.

Installation of chained root certificates are more complex and some web servers are not compatible with chained root certificates.

For a Certification Authority to have its own Trusted Root CA certificate already present in browsers is a clear sign that they are long-time, stable and credible organizations who have long term relationships with the browser vendors (such as Microsoft and Netscape) for the inclusion of their Trusted Root CA certificates. For this reason, such CAs are seen as being considerably more credible and stable than chained root certificate providers who do not have a direct relationship with the browser vendors.

Chained root certificates require additional effort to install as the webserver must also have the chained root installed. This is not necessary for single root certificates.

Both FreeSSL.com's ChainedSSL Wildcard product and Comodo's InstantSSL product are chained root certificates. However FreeSSL.com own the trusted CA root used to issue ChainedSSL and are therefore the only stable chained root provider. Comodo do not own the BeTrusted root used to issue InstantSSL certificates and therefore cannot offer the stability of ChainedSSL or our single root certificate StarterSSL.
These FAQs are all courtesy of FreeSSL.com!