All internet communication between your computer and the website can be intercepted by your network administrator, ISP or any other party between you and the website. A large number of really simple network tools are available which make it easy to decipher communication between your computer and the Internet.
That is why when you access your online bank accounts on internet or any other confidential site, the server forces you to use https instead of http in the URL. Almost all websites asking for credit card or other personal information use https. Infact you should avoid putting your confidential information on a website page if it is not using https.
Though there is no need to get paranoid about security, situations do arise when you should know how to protect your internet communication. One of the popular applications on internet is email. If you are like me and use Gmail, you can force Gmail to use https instead of http to login and read and write your emails securely.
By default, Gmail uses https only for your login information. Rest of the communication (reading, writing emails) is not protected. But if you start your login process with https (e.g. https://mail.google.com/, Gmail will keep you connected through https. This is true for all google services including google apps.
Not all websites provide secure access through https which is ok most of the time. However you can investigate or ask the webmaster of a particular site for https access if you are sending any confidential
information over the Internet.