Answer 1

getchar() reads standard input and returns one char from it.

gets() reads a string from stdin (But don't x it, use fgets instead)

Ej.:
#include <stdio.h>
#include <stdlib.h>
int main() {
    char grade;
    char name[20];

    printf("input grade (a/b): ");
    grade = getchar();

    fflush(stdin);

    printf("input name: ");
    gets(name);

    printf("%c - %s\n", grade, name);

    return 0;
}

Answer 2

https://www.sololearn.com/learn/C/2914/

getchar() is used when you read one single character from input stream.

gets() is used when you read an array of characters.

Hint: You can play with the Try It Yourself in each lesson a little bit, try changing the numbers/variable names/datatype and see what happens