What is an SSL certificate?

SSL stands for Secure Sockets Layer. It is the basic technology to establish a coded connection between a client, your internet browser and a server. In order to establish an SSL connection, the server needs an SSL certificate. The exchanged data are encrypted and can only be read by the receiver. Data cannot be intercepted or modified (e.g. for online payments).

Identity

In addition to data encryption, the SSL certificate is also used as digital identity card. Obtaining an SSL certificate, recognized by a Certificate Authority, a publisher of a certificate proves to the visitor that the address data of the domain name are authentic. When applying for a certificate you always need to submit the necessary official documents proving your identity (e.g. identity card).
Websites with a valid SSL certificate offer guarantees against fraud and help preventing phishing.

What does a certificate contain?

A certificate is a special text file consisting of 2 parts: a readable text part containing information on the owner, the publisher of the certificate, etc. and a second encrypted part (non readable to people) containing the digital signature and a public key of the certificate authority. The text file has a .cer-extention so that the operating system can open the file with a (built-in) application to read certificates. If you would open a certificate in a text editor, it would look like this: 

-----START OF CERTIFICATE-----

CBHcm91cCBDQS5jcmwwR qBEoEKGQGZpbGU6Ly9cX ENFUlRTUlZcQ2VydFNyd lxDZXJ0RW5yb2xsXE1TI ENlcnRTcnYgVGVzdCBHc m91cCBDQS5jcmwwCQYDV R0TBAIwADBiBggrBgEFB QcBAQRWMFQwUgYIKwYBB QUHMAKGRmh0dHA6Ly9DR VJUU1JWL0NlcnRTcnYvQ 2VydEVucm9sbC9DRVJUU 1JWX01TIENlcnRTcnYgV GVzdCBHcm91cCBDQS5jc nQwDQYJKoZIhvcNAQEEB QADQQAhq70nRlse0ulPs tU+IWdjeNj5p

-----END CERTIFICATE-----