When you use a web browser the information is usually sent unencrypted. That means anyone can read the traffic between you and the server. 

It is not in princip necessary to encrypt everything that is transferred over the internet, but when you visit e.g. your net banking account it is of highest importance.

HTTPS is an encrypted version of the HTTP protocoll and when you go to your net banking system you will notice the address begins with HTTPS.

  • Whenever you do online shopping, make sure the address starts with HTTPS

All respectable companies have HTTPS for sensitive data transmission. Please avoid all other shops.

Nowadays HTTPS is becoming quite popular among different kinds of web sites. E.g. reddit.com and start.arcada.fi both use HTTPS.