putchar() / getchar() / puts() / gets_s()

// Writes a single character to standard output (equivalent to fputc(c, stdout)).
// Returns: the written character on success, EOF on error.
int putchar(int c);

// Reads a single character from standard input (equivalent to fgetc(stdin)).
// Returns: the character read as an int, or EOF on end-of-file or error.
int getchar(void);

// Writes a string to standard output and automatically appends a newline.
// Returns: a non-negative value on success, EOF on error.
int puts(const char *str);

// Reads up to n-1 characters from standard input (safe replacement for gets(), C11 and later).
// Returns: str on success, NULL on failure.
char *gets_s(char *str, rsize_t n);

#include <stdio.h>

int main(void) {
    // Output strings with puts() — a newline is appended automatically.
    puts("Hello, World!");   // Outputs "Hello, World!" followed by a newline.
    puts("Learning C!");     // Outputs on the next line.

    // Output characters one at a time with putchar().
    putchar('A'); // Outputs "A".
    putchar('B');
    putchar('C');
    putchar('\n'); // Outputs a newline.

    // Read a character with getchar() and check whether it is uppercase or lowercase.
    printf("Enter a character: ");
    int ch = getchar(); // Waits for input.
    if (ch >= 'a' && ch <= 'z') {
        printf("Lowercase: %c\n", ch);
    } else if (ch >= 'A' && ch <= 'Z') {
        printf("Uppercase: %c\n", ch);
    } else {
        printf("Not a letter: %c\n", ch);
    }

    return 0;
}

puts() automatically appends a newline ('\n') at the end of the string. To do the same with printf() you must include '\n' explicitly, so puts() saves you that step. It is not a good fit, however, when you need output without a trailing newline.

Always store the return value of getchar() in an int variable. If you store it in a char, on platforms where char is signed, EOF (-1) can be confused with the value 255, causing an input loop to never terminate correctly.

gets() is a dangerous, deprecated function. It was removed from the C standard in C11 because it provides no way to specify a buffer size, making buffer overflows inevitable. Use gets_s() (C11 and later) or fgets() instead.

For reading from and writing to files, use fgets() / fputs() / fgetc() / fputc().