Should I use window.variable or var?

We have a lot of setup JS code that defines panels, buttons, etc that will be used in many other JS files.

Typically, we do something like:

grid.js

var myGrid = .....

combos.js

var myCombo = .....

Then, in our application code, we:

application.js

function blah() {
    myGrid.someMethod()
}

someother.js

function foo() {
    myCombo.someMethod();
    myGrid.someMethod();
}

So, should we be using the var myGrid or is better to use window.myGrid

What's the difference?