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Every browser comes with a pre-defined list of well-known CAs. You can find a sample list of CAs at http://www.dmoz.org/Computers/Security/Public_Key_Infrastructure/PKIX/Tools_and_Services/Third_Party_Certificate_Authorities/.
Along with the name of your organization and the name of your server, a CA-signed certificate contains the public key of the server. This public key is used by the browser to encrypt data sent to the server. There is a private key on the server. The server uses the private key to decrypt the data encrypted by the public key. The private key should be kept secure on the server to prevent unauthorized access.
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