Strings aren't arrays, arrays aren't strings.
What's the difference between a basketball and a barbequeue?
What he said. They have absolutely nothing in common.
Maybe you meant what is the difference between a Variable and an Array?
Because essentially an Array is a variable, which can contain a multiple number of expressions.
But, to answer what you're asking, a String is like a sentence, for example:
"This is a string."
An Array is exactly what I stated above, it's a variable that contain multiple numeric values, or technically I guess it could be used to store anything, and more than one!
For example:
Normal Variable -
Array -
Maybe you saw something like this:
String months[] =
{"Jan", "Feb", "Mar", "Apr", "May", "Jun",
"July", "Aug", "Sep", "Oct", "Nov", "Dec"};
And it confused you possibly?