Layouts determine how all the UI elements in a window should be placed. All menus and windows you see in Maya are created in MEL using specific UI commands such as layout, slider, field, button, etc!
There should be many cases when you must be wondering or getting frustrated to figure out the right UI element or layout? For this reason Matt created a really useful resource on documentation for UI elements so that we can pick the right one without much of a hassle. Click Here for the PDF documentation! Here is the original post from Tech-Artist.org.
Also check out this documentation on creating MEL UI Layouts. Useful resource for UI creation. 🙂