A Certificate Authority, or CA, is a reliable third party authorized to issue and manage Digital Certificates that are used in secure (SSL) connections.
The CA verifies the identity of the business or institution requesting a Digital Certificate, and the services that they are authorized for. In doing so, the CA vouches for a company's identity and presence (both physical, and on the web) to existing and prospective Customers. To do so, the CA provides the requester a digital certificate that they can put on their website, to enable secure data transmission. This digital certificate has become a crucial part of E-commerce today.
Popular Certificate Authorities include Verisign, Thawte, etc.