I'm trying to understand getchar() != EOF

I'm reading The C Programming Language and have understood everything so far. However when I came across the getchar() and putchar(), I failed to understand what is their use, and more specifically, what the following code does.

main()
{
    int c;
    while ((c = getchar()) != EOF)
       putchar(c);
}

I understand the main() function, the declaration of the integer c and the while loop. Yet I'm confused about the condition inside of the while loop. What is the input in this C code, and what is the output.

This code can be written more clearly as:

main()
{
    int c;
    while (1) {
        c = getchar();            // Get one character from the input
        if (c == EOF) { break; }  // Exit the loop if we receive EOF ("end of file")
        putchar(c);               // Put the character to the output
    }
}

The EOF character is received when there is no more input. The name makes more sense in the case where the input is being read from a real file, rather than user input (which is a special case of a file).

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[As an aside, generally the main function should be written as int main(void).]