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In computing, a command is a directive to a computer program acting as an interpreter of some kind, in order to perform a specific task. Most commonly a command is a directive to some kind of command line interface, such as a shell.
Specifically, the term command is used in imperative computer languages. These languages are called this, because statements in these languages are usually written in a manner similar to the imperative mood used in many natural languages. If one views a statement in an imperative language as being like a sentence in a natural language, then a command is generally like a verb in such a language.
Many programs allow specially formatted arguments, known as flags, which modify the default behaviour of the command, while further arguments describe what the command acts on. Comparing to a natural language: the flags are adverbs, whilst the other arguments are objects.
The following command prints the text hello out to the standard output stream, which, in this case, will just print the text out on the screen. echo is the command and "Hello World" is the argument. The quotes are used to prevent Hello and World being treated as separate arguments:
echo "Hello World"
ls -l -t -r /bin ls -ltr /bin
The following command displays the contents of the files ch1.txt and ch2.txt. cat is the command and ch1.txt and ch2.txt are both arguments.
cat ch1.txt ch2.txt
The following command lists all the contents of the current directory. dir is the command and "A" is a flag. There is no argument. Here are some commands given to a different command line interpreter (the DOS, OS/2 and Microsoft Windows command prompt). Notice that the flags are identified differently but that the concepts are the same:
The following command displays the contents of the file readme.txt. type is the command. "readme.txt" is the argument. "P" is a parameter...
type /P readme.txt
See also 
|Wikibooks has a book on the topic of: Guide to Windows commands|