The Internet is all about various computers communicating with each other. The prevailing model of the Internet is the notion of clients and servers. You can understand this better by imagining a drive-through restaurant. As you drive to the little speaker, a barely intelligible voice asks for your order. You ask for your "cholesto-burger supreme," and the bored teenager packages your food. You drive up, exchange money for the combo meal, and drive away. Meanwhile, the teenager waits for another customer to appear. The Internet works much like this model. Large permanent computers called Web servers permanently host Web pages and other information. They are much like the drive-through restaurant. Users "drive up" to the Web server using a Web browser. The data is exchanged, and the user can read the information on the Web browser.
The server is also a computer, and it's possible to write programs designed to operate on the server rather than the client. There are a number of advantages to this arrangement:
Server-side programs run on powerful Web server computers.
The server can freely work with files and databases.
The code returned to the user is plain HTML, which can be displayed on any Web browser.