Layouts in GTK
GTK utilizes a layout system that differs from many other desktop toolkits. While other toolkit's may prefer users to place widgets on a specific pixel point (a button at [50, 50]), GTK uses a flexible container-based system similar to HTML5 on the web. Containers in GTK are widgets that can "hold" other widgets. Each container uses a slightly different method to display its children (the widgets it is "holding").
Calculated Space
One important concept in GTK is calculated space. Widgets by default take up the space that they are calculated to need to display their content and nothing more. This behavior can be overriden using containers and individual settings within the widgets.
Gtk.Box
Gtk.Box is the simplest of all the containers. It arranges its child widgets either vertically or horizontally in a simple linear line. Children can either be homogenous (receiving equal space) or vary depending on their calculated space.
Gtk.Grid
Gtk.Grid is, essentially, a two dimensional, homogenous Gtk.Box. Its child widgets are arranged in neat rows and columns.
Gtk.FlowBox
Gtk.FlowBox is similar to HTML5's Flexbox conceptually. Gtk.FlowBox fills a row or column with child widgets (depending if it is set to vertical or horizontal) until there is no space left before starting a new row. This makes Gtk.FlowBox the most flexible of all the containers as you are not limited by predefined rows, columns, or sizes.
Gtk.ListBox
Gtk.ListBox is another flexible container. It does not have a predefined number of elements. Gtk.ListBox unlike other containers only accepts one type of child widget Gtk.ListBoxRow. However, Gtk.ListBoxRow can contain any widget you choose.