There are three different types of SSL (Secure Sockets Layer) certificates publishers may employ:
- Domain Validated (DV) Certificate
- Organization Validated (OV) Certificate
- Extended Validation (EV) Certificate
The green "secure" address bar and accompanying padlock require the use of Extended Validation (EV) SSL certificates. EV certificates are the most thorough, secure, and therefore expensive certificates available.
Publishers using either DV or OV certificates will have a grey padlock displayed, without the green "secure" graphics or color.
For more information regarding the different types of SSL certificates, see this article.