An integer (more commonly called an int) is a number without a decimal point.
ex. any whole number. 74 69 69 63 22 -31

A float is a floating-point number, which means it is a number that has a decimal place. Floats are used when more precision is needed.
ex. 13.1415 77.9999 69.6969
a double has 2x the precision of float. In general a double has 15 to 16 decimal digits of precision, while float only has 7.

string is a sequence of characters. A string is noted by surrounding a group of letters with double quotes. Chars and strings are most often used with the keyboard methods, to display text to the screen, and to load images or files.