A global variable is any variable declared outside of a function, while a local variable is any variable declared inside of a function. These are the only two types. A global variable can be used anywhere, but a local variable can only be used/called upon inside of its function.
Math operators may be used to add numbers or strings together or to add, subtract, etc, variables.
So, you can set a variable equal to 5 + 6, and the value of the variable will be 11. Or, you can set a variable "C" equal the sum of variable "A + B" (lets say A's value is 5 and B is 18), so that C's value is now 23.
In modulus division, only the remainder of a solution becomes the value. So if you set "var a = 21 % 5;" a would equal 1. This is because 5 goes into 21 4 times (which is not taken note of) and then has 1 leftover. Modulus division can be very very useful, but that is all the time we are going to spend on it right now.
There are also such things as increment and decrement operator. To add on to a variable, just write the variable name followed by to addition signs. Here is an example: "var1 ++;" The same thing works for "--," the variable will decrease by 1.