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In UNIX/linux programming, the system("...") command is used in C/C++ code to execute a command as if you typed it at the command line.
For example:

system("cat readme.txt");
would print the readme.txt text file out to the screen just as if you had typed "cat readme.txt" on the command line.

It is a very bad idea to use this command in programs that will be running with special (or really any) priveliges (for security reasons). By hex editing or devious file name and path manipulation, a malicious user (hacker) with access to that program could easily execute anything he or she desired on your machine.
A particularly good reference to learn more about this and other unix commands is Advanced Programming in the Unix Environment by W. Richard Stevens.