what library to include

I am a novice at SAPUI5. As a first step I added a GridLayout to my code. This required the addition of a library. GridLayout is a member of sap.ui.layout.form. I added this library and copied from the demokit sample

var oLayout1 = new sap.ui.layout.form.GridLayout();

This gave an error saying GridLayout is not a constructor. The API documentation does not allow for an empty constructor but this is the way the sample code  uses it.

A colleague suggested I use library sap.ui.layout. Despite the fact this lacks a GridLayout it worked. Can someone explain why? How would I know to use that library rather than the one that actually contains the GridLayout?

Thanks

Cliff