In many web applications the use of the secure socket layer (SSL) protocol is required. If you are a frequent user of e-commerce sites, you are undoubtedly familiar with the concept - basically, the URL of the page will start with https:// (as opposed to the "normal" http://) and the browser will encrypt all information sent to the site.
But not only! SSL also plays an important role as a mechanism for authenticating your data's destination. When you enter your credit card number while buying books, for example, you want to make sure not only that the information is encrypted, but also that you are submitting your card to a real bookstore (let's say, Amazon.com) and not to some illicit web site which imitates it.