Most operating systems (The MacOS of the '80s and '90s being a partial exception) offer a command line interface to a user. This provides a means for the user to type instructions for the computer to carry out. Most users hate this procedure because it requires them effectively to learn a computer language in order to communicate with the machine. Sometimes commands offer switches that are difficult to remember, and sometimes the command names themselves are hard to associate with their functions.
On the other hand, there are a number of advantages to using command interfaces - indeed, experienced computer users prefer them.
There is a summary of many of the most often used commands in W2k and Unix in the PDF Text.