SSL is an acronym for Secure Sockets Layer and what it does is encrypt the traffic going from your browser to the Web Server.
SSL is really no longer a thing and has been replaced by TLS or Transport Layer Security. It is just still referred to as SSL for simplicity and old habits die hard as it were.
Do to many regulations if you have any type of login or PII (Personal Identifiable Information) you maybe legally required to have an SSL to encrypt all of your website traffic or at least the the parts with logins and sensitive information.
If you do not have a SSL your web traffic will not be encrypted and a nefarious hacker can sniff the traffic and steal this information.
The things that often define price are:
- Encryption Level aka 128 bit, 256 bit, etc..
- How many domains/subdomains does it cover
- The Name i.e. Symantec which are great SSL as offer many Benefits and one of the many reason people like and get them is because it is a name that everyone knows.
- Many SSLs offer some type of insurance in case there are legal issue do to there SSLs encryption.
You can get some SSLs for free, the ones you get from your web host are fine. The there are some others out there that are questionable.
One you do not usually want to use for External Website Traffic is a Self-Signed SSL by your server.
These type of SSLs are fine for Internal websites that are on a company website or server to server communication.