ROS 2 Jazzy Jalisco

In ROS 2,  your "workspace," refers to the place on your computer where you're doing your ROS 2 development. 

Think of a workspace as your personal project area. There are two main types of workspaces in ROS 2; the core workspace, known as the "underlay," and any additional workspaces you create, called "overlays." It's common to work with multiple workspaces at the same time.

Using multiple workspaces is helpful for a few reasons. It allows you to easily develop and test against different versions of ROS 2 or various sets of packages. 

You can also have multiple ROS 2 distributions (like Humble and Iron) installed on the same machine and switch between them as needed.

To make this work, you need to "source" the setup files whenever you open a new terminal. Sourcing these files sets up the environment so that ROS 2 commands and packages are available to you. You can do this manually each time you open a terminal or add the source command to your shell's startup script to automate the process (This is not recommended when you are working with multiple ROS2 distributions)

Overlay:
Custom Packages: Packages created by the user, which can be specific applications or nodes.

Underlay:

Core ROS2 Packages: Fundamental ROS2 packages providing essential functionalities.

Dependencies: Libraries and other dependencies required by ROS2 and its packages.

In practice, the Overlay builds on top of the Underlay, allowing for custom developments while utilizing the core functionalities provided by ROS2.