int: Short for integer, just a type for containing basic whole number values.
var: Short for variable, not used anywhere in AGS.
string: A type used to hold "strings" of characters (letters), this text will be held in a string somewhere in your computer.
String: The new type of string in AGS 2.71+, similar, just easier to use.
float: Used in a similar way to int, but can hold floating point numbers, like 3.142
bool: A variable type that can only contain two logical values, true or false.
If you want to store a number somewhere you can do so in a variable of type "int", like
myVariable = 30;
Read through any programming tutorial for how to use variables.