The Internet relies on a number of protocols in order to function properly. A protocol is simply a standard for enabling the connection, communication, and data transfer between two places on a network. Here are some of the key protocols that are used for transferring data across the Internet.
HTTP stands for Hypertext Transfer Protocol. It is the standard protocol for transferring web pages (and their content) across the Internet.
When you browse a web page, the URL might be preceded by http://. This is telling the web browser to use HTTP to transfer the data. Most browsers will default to HTTP if you don't specify it. You can test this by typing in say... www.quackit.com (instead of http://www.quackit.com).
HTTPS stands for Hypertext Transfer Protocol over Secure Socket Layer. Think of it as a secure version of HTTP. HTTPS is used primarily on web pages that ask you to provide personal or sensitive information (such as a password or your credit card details).
When you browse a web page using HTTPS, you are using SSL (Secure Sockets Layer). For a website to use HTTPS it needs to have an SSL certificate installed on the server. These are usually issued by a trusted 3rd party, referred to as a Certificate Authority (CA).
When you browse a web page using HTTPS, you can check the details of the SSL certificate. For example, you could check the validity of it. You could also check that the website does actually belong to the organization you think it does. You can usually do this by double clicking on the browser's padlock icon. The padlock icon only appears when you view a secure site.
Here's an example of Firefox's padlock icon from Firefox's address bar:
Firefox also displays a padlock at the bottom right of the browser window, along with the common name of the organization that the certificate was issued to:
FTP stands for File Transfer Protocol. It is used to transfer files across the Internet. FTP is commonly used by web developers to publish updates to a website (i.e. to upload a new version of the website).
Where HTTP is used for displaying the file in your browser, FTP is used simply to transfer the file from one computer to a specified location on another computer. You can use FTP to transfer the files from your computer to a remote computer (such as a web server), or to transfer from the remote computer to your local computer.