Basically, https is a certificate that states that you're dealing with the real website and there's no middleman in between. You can find some more history here: http://en.wikipedia.org/wiki/Transport_Layer_Security
HTTP and HTTPS is the difference between keeping you secure. More information can be found at articlesbase.com/internet-articles/what-is-the-difference-between-http-and-https-72618.html
Another interesting questions is the difference between SSl certificates. There are plenty of SSL products and most people do not understand the differences very well.
Basically, you'll need to install an SSL certificate on your site if you wish to accept payments on your site via Credit card or payment gateways (E-commerce site).
SSL will keep your traffic secure and will prevent hackers from sniffing the credentials of your users, or any other sensitive information that is transmitted to/from your site. It also gives more confidence in your side to your users.
SSL requires a certificate and has to be purchased, although you may be able to find free one's but they are usually unsigned. Well SSL cannot prevent sniffing traffic 100% because if there is a keylogger on the computer, they can see anything that is typed such as login and passwords.. SSL is really just required for sites that sell and buy things.