When you buy a SSL certificate from Comodo, it is signed by Comodo and therefore trusted. How is it signed though? Why can't anyone sign the certificate pretending to be Comodo?
Comodo have a root certificate whose public key is included in your web browser. The private key that matches the public key is used to sign SSL certificates that Comodo issue, and it can't be faked because no one else has the private key that matches the public key in the web browser.