In C you might find yourself using double quotes in some cases, and single quotes in other cases.
In some languages, there's no difference between the two. But in C, there is difference, and you'll use one vs the other depending on the need.
When can you use single quotes and when can you use double quotes?
Single quotes are used to identify a single character (char value):
char letter = 'a';
Double quotes are used to create a string literal:
char *name = "Flavio";
Note that you can create a single-letter string literal:
char *letter = "a";
But remember that a string is composed by the characters of the string, plus a 0 character at the end, which makes single-letter string literals take double the space of a single character.