What is the difference between ' ' and " "? [duplicate]

This question already has an answer here:

• Single quotes vs. double quotes in C 11 answers

I was creating a simple program to count the words and vowels of a string, and when I try to use double quotes inside of a 'for' or an 'if' statement it gives an erro. But when I change it for a single quote it works pretty good.

I thought they were the same thing, so what is the difference between them and why I cannot use double quotes inside of a statement or single quote inside a 'printf' function?

PS: I'm using code blocks as my IDE.

Here is my code, so you can see an example:

#include <stdio.h>
#include <conio.h>

int main(void){
    // Declaracao de Variaveis
    int cont_p = 0, cont_v = 0, i;
    char frase[1000];

    // Leitura de Dados
    printf("Digite a frase:\n");
    gets(frase);

    // Logica e Contagemm
    for (i=0; frase[i]!='\0';i++){
        if(frase[i] == ' '){
            cont_p = cont_p +1;
        }else if ((frase[i] == 'a') || (frase[i] == 'A')) {
            cont_v = cont_v + 1;
        }else if ((frase[i] == 'e') || (frase[i] == 'E')) {
            cont_v = cont_v + 1;
        }else if ((frase[i] == 'i') || (frase[i] == 'I')) {
            cont_v = cont_v + 1;
        }else if ((frase[i] == 'o') || (frase[i] == 'O')) {
            cont_v = cont_v + 1;
        }else if ((frase[i] == 'u') || (frase[i] == 'U')) {
            cont_v = cont_v + 1;
        }

    }
    cont_p++;

    // Exibindo o Resultado
    printf("\n\nNumero de PALAVRAS: %d\n", cont_p);
    printf("Numero de VOGAIS: %d\n\n\n", cont_v);
    printf("Programa Finalizado!\n");
    getch();
    return 0;
}