An SSL (secure sockets layer) certificate is a digital certificate that both authenticates the identity of a website, and encrypts sensitive information so that any passwords, addresses or credit card numbers can not be intercepted or read by anyone other than the intended recipient
When you use a website http messages are flying around over the network. When you fill in a contact form or simply click a link a small packet of information in text format gets sent over the network by your computer. If you put your email address into a contact form and hit submit the packet of information (very similar to a plain text file) will contain your email address within it. This packet will then get sent to every machine on the network. If you happen to be using wireless then this information will be sent over the air.
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