Let's create a catkin workspace:

$ mkdir -p ~/catkin_ws/src
$ cd ~/catkin_ws

Even though the workspace is empty (there are no packages in the 'src' folder) you can still "build" the workspace:

$ catkin_make

The catkin_make command is a convenience tool for working with catkin workspaces. If you look in your current directory you should now have a 'build' and 'devel' folder. Inside the 'devel' folder you can see that there are now several setup.*sh files. Sourcing any of these files will overlay this workspace on top of your environment. To understand more about this see the general catkin documentation: catkin. Before continuing source your new setup.*sh file:

$ source devel/setup.bash