The Internet, boiled down to its most basic description, is a series of computers that are all connected through a series of Transmission Control Protocols (TCP) and Internet Protocols (IP). This network is better known as the TCP/IP network. The Internet was originally called ARPANet and was created by the United States government in the 1960s, and improved upon in the 1970s. ARPANet's original task was communicate between different branches of government and installations in the event of a nuclear war.
When the Internet transformed into more of a commercial venture in the 1980s, most home computer users connected to the network using a dialup modem, which transmitted specific data packages over phone lines to connect users to the network. Over the last two decades, dial-up modems have become outdated as most Internet connections use broadband cable and mobile devices to connect via wireless networks.
While most people tend to think that the Internet and the World Wide Web are one and the same, the Web is actually a way for people to access information while using the Internet. Instead of TCP/IP, the Web uses Hypertext Transfer Protocols (HTTP) in order to transmit data and share information with other web pages that use the same protocols as a sort of Internet language. The Web, not the Internet, uses browsers such as Firefox, Internet Explorer and Google Chrome to access websites and webpages. Search engines allow people to use the Web to find specific webpages that are coded to certain Web addresses.
While the Web is one way in which to communicate using the Internet, there are others such as email and instant messaging that may be confused as being part of the Web but is actually a separate entity also utilizing the Internet in order to communicate with various servers. The easiest way to keep the Web and Internet separate is to realize that the Web communicates primarily with HTTP language while the Internet can also use SMTP for email messaging and FTP for instant messaging.
The best way to think of the Internet is as the backbone behind all your favorite sites, rather than any group of sites themselves.