SSL stands for Secure Socket Layer it ensures that data travelling between you and the website is secured and that if someone was to try and steal that data would be very difficult for them to do so.
When you are paying for goods online it is absolutely essential to ensure that your customers are protected, even if they are paying through a payment gateways it is usually best to have SSL anyway.
Standard
This is your general certificate you will mostly use. It will work with only one domain and one subdomain.
Wildcard
This will allow you use the certificate on any sub domain on your domain.
Multi
We do not recommend this one but it allows multiple sites on one certificate.
Then under those the level of validation is as follows:
Also please note that the company and extended validation are considerably more expensive than the domain validated as there is more processes behind them.