To have a website secured with SSL, the administrator needs to generate some files, send them off to an authority, get more file back and put them on the server where they are used to provide the security of SSL.
Obvious. But the questions is, “what are these files?”.
First you need to generate the CSR file and the KEY file. CSR is short for “Certificate Signing Request”. KEY is just a key file which remains private and you don’t give it to anyone. The CSR file is what you provide to the certificate authority such as Comodo for example.
Once you’ve provided the CSR file to the authority, you will (shortly later) receive two files back. The CRT and BUNDLE. CRT stands for “Certificate”. The Bundle is not always included but should be. It makes everything fit together nicely.
Now we have four files. The CSR, KEY, CRT and BUNDLE. The CSR is not used any further. Keep it safe somewhere. The KEY, CRT and BUNDLE are put on the web server in a safe location. Those three are used by the web server software (such as Apache or NginX) to provide the SSL capability.
So you keep the KEY safe away from the server. Put the CSR, CRT and BUNDLE on the web server and you’re done.