Understanding closures seems to be a rite of passage for javascript engineers. Here is the simplest closure:

A closure is a function along with the data contained by the scope in which that function was defined.

A closure is created whenever a javascript function is created. The function keeps a reference to its parent scope so that it is able to access other variables and functions defined in that scope.

// parent scope
var home = 'Moscow';

function whereIsHome() {
    // function scope
    alert(home);
}

whereIsHome();    // alerts 'Moscow'

Here the variable home is outside of whereIsHome’s function scope.

whereIsHome keeps a reference to its parent scope, the scope where it was defined.

When whereIsHome is called, it can access home.