An SSL certificate is digitally "signed" by a certificate authority, such as GoDaddy or Thawte, that web browsers already trust. This allows the web browser to verify the identity of a secure site before sending private personal information, such as bank account or credit card numbers. Webmasters can purchase certificates from the certificate authorities, which verify the webmaster's identity to varying degrees.
For a detailed explanation of certificates and how they actually work, see what is SSL? For help deciding which SSL certificate to buy, see my articles which SSL certificate should I buy? and can I set up a secure site without buying an SSL certificate?
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