This course teaches C under the UNIX operating system. C programs will look similar under any other system (such as VMS or DOS), some other features will differ from system to system. In particular the method of compiling a program to produce a file of runnable code will be different on each system.
The UNIX system is itself written in C. In fact C was invented specifically to implement UNIX. All of the UNIX commands which you type, plus the other system facilities such as password checking, lineprinter queues or magnetic tape controllers are written in C.
In the course of the development of UNIX, hundreds of functions were written to give access to various facets of the system. These functions are available to the programmer in libraries. By writing in C and using the UNIX system libraries, very powerful system programs can be created. These libraries are less easy to access using other programming languages. C is therefore the natural language for writing UNIX system programs.