That's what virtualization allows businesses to do. One computer can be "chopped" into many bits, usually referred to as a virtual machine, or VM. Each VM can run its own OS and have a specific task, such as web serving, or providing email functionality.

Previously a computer could run any number of programs at the same time, of course, but virtualization is fundamentally more secure because each OS is confined to its own VM. If a hacker compromises a program running on one VM, he won't have access to the other programs running on the other VMs.

In terms of costs and overhead, not only is there only one computer to administer but electricity bills are also reduced and there's no need to provide as much air conditioning in data centers. (Typically each running as many virtualized operating systems as they can get onto them bearing in mind RAM, CPU and storage limitations, but this is still cheaper than each having its own computer.)

Data centers

You can run virtual machines on your own server computers in your own premises, of course, but lots of people either install or rent physical computers in data centers and run them there. They then access them across the Internet.