IMGUI is the standard GUI drawing method for all Editor GUI up until 2020.1 (UIToolkit is now recommended by Unity). IMGUI stands for Immeditate Mode GUI, this means that it is redrawn every frame. This makes it really easy to use for quickly creating new UIs and constantly the visual representation with the data. However because of this IMGUIs are often relatively expensive to render as it is hard to do virtualization and to only redraw elements that have changed.
Unity supports 2 different layouting methods for IMGUI. The automatic or GUILayout method, as the name implies tries to automatically place the elements based on hints given by the developer. The other is the manual layouting mode, this lets you place elements at specific screen coordinates and no automatic layouting or sizing is done on these elements.
In most cases you won't have to mix these methods however it is useful to know as not all controls are available in both methods (mostly GUI methods not being available in GUILayout).
As previously mentioned, the automatic layout method tries to place the UI elements based on hints provided by the developer. This makes it easier to quickly create new UIs in a similar style in exchange for a little less control. It starts by placing controls in the top left corner.
All automatically layouted controls are defined in the (Editor)GUILayout class, most general purpose controls are defined in GUILayout. Some have an equivalent in EditorGUILayout, these often have extra functionality specific to the editor. For example they respond correctly to actions such as select all, copy, paste, and they can also have an optional label in front.
There are several ways to influence the placement of GUILayout controls, most often you'll be using Horizontal (Begin/EndHorizontal or HorizontalScope) and Vertical (Begin/EndVertical or VerticalScope) groups, these allow for most basic UIs. You can also pass GUILayoutOptions to most controls, these allow you to change that control specifically, it's a bit counter intuitive however these are also defined in GUILayout (e.g. GUILayout.Width or GUILayout.Min/MaxWidth).
Placing controls at either the right or bottom side can be done by using a layout group and the GUILayout.FlexibleSpace control, which will take up all leftover space.
As you might expect the manual layouting method requires you to manually define the location of the controls, this gives you a lot of control in exchange for a bit of extra work. Manual layouting also has higher performance than GUILayout, in my opinion it does not matter which you use for most editors. Keep in mind that you are forced to use manual layouting when creating PropertyDrawers.
Similarly to the automatic layouting method there is also a EditorGUI class with editor specific controls.