In C, a “stream” is an abstraction; from the program’s perspective it is simply a producer (input stream) or consumer (output stream) of bytes. It can correspond to a file on disk, to a pipe, to your terminal, or to some other device such as a printer or tty. The FILE type contains information about the stream. Normally, you don’t mess with a FILE object’s contents directly, you just pass a pointer to it to the various I/O routines.
There are three standard streams: stdin is a pointer to the standard input stream, stdout is a pointer to the standard output stream, and stderr is a pointer to the standard error output stream. In an interactive session, the three usually refer to your console, although you can redirect them to point to other files or devices:
$ myprog < inputfile.dat > output.txt 2> errors.txt
In this example, stdin now points to inputfile.dat, stdout points to output.txt, and stderr points to errors.txt.
fprintf writes formatted text to the output stream you specify.
printf is equivalent to writing fprintf(stdout, ...) and writes formatted text to wherever the standard output stream is currently pointing.
sprintf writes formatted text to an array of char, as opposed to a stream.