A desktop is like a private (personal) computer.  You can call it a PC.  People really needed private computers about 20 years ago.  A PC consists of an input, and an output space, with a screen for it.  Things such as disk drives and printers help make the PC way more versatile.  You know, by adding more input and output, the stuff that makes the PC communicate with other private/personal computers.

In the year 1642, the French mathematician and philosopher, Pascal, came up with a calculating device which would be called the "Adding Machine."  He was the 18-year-old son of a tax collector.  He wanted to invent something to help his dad in his work.  Originally called a "numerical wheel calculator" or the "Pascaline", that was when the computer had its beginning.  In the 20th century people were still looking for a computer that could do all of their tasks.  The Mark I was a general computer that he invented a long time ago that first did multiple tasks.