A 'string' is a sequence of 0 or more characters.  That means that strings can be used to represent words, sentences, an entire document or single characters.  In general if you know you are only going to use a single character you should use the 'char' type as it's more efficient.

Difference between "a" and 'a':
When you assign a value to a string variable you must use double-quotes around the value like: "Hello World".  Double-quotes tells the compiler that the constant "Hello World" is a string and not a char.  That might be obvious in this case since more than one character is being used, you couldn't assign "Hello World" to what's the big deal?  Well you can assign a single character to a char and you can also assign a single character to a string, so how does the compiler know the difference?  It knows because you tell it with the types of quotes you use.  Using single quotes means the constant is a char like: 'H'.

char letter = 'a';      // OK
char letter = "a";      // ERROR: can't assign a string to a char

string word = "a"       // OK
string word = 'a'       // ERROR: can't assign a char to a string

str( ) function:
Often you will need to convert a variable of some type to a string to display in a message to the user, or to save to a file.  You can convert any built-in type to a string with the str() function.  This function takes as it's argument any variable and returns a string representation of that variable.

int  showDouble(double num)
{
    string msg = str( num );

    PostMessage( "Your number is: " + msg );
}

This function displays a message containing the number the user passed to it.  For example if you passed it the number 2.71828 the output would be:

Message: Your number is: 2.71828

10 bonus points to anyone who knows what the number is.