An SSL is the technology behind two computers (host and client) that are encyptedly connected. Source
It is a sort of online ID that ensure that a company is who they say they are by researching companies, checking their references, and assuring a customers identity. Most importantly it also encrypts data (such as payment information). Source
Symantec, GeoTrust, Comodo Source
A server, your WHOIS record & the CSR from the server are all needed to set up an SSL, once you have all these you submit your CSR to to organization that you want to provide you with an SSL, and when that's validated tehy will send you an SSL for you to install. Source
Midphase is the server that I use, and they provide SSL's through Comodo and PositiveSSL that's 99.3% compatable with browswers, 128-bit encryption, and shows a lock on your browser. This is an ad-on to hosting that jumps the price by $20, which now makes sense why my hosting is so cheap! Source