By running tmux with no arguments, you create a brand new session. In tmux, the base unit is called a session. A session can have one or more windows. A window can be broken into one or more panes. We'll revisit this topic in its own whole chapter (for more information, refer to Chapter 3, Sessions, Windows, and Panes); however, as a sneak preview, what we have here on the current screen is a single pane taking up the whole window in a single session. Imagine that it could be split into two or more different terminals, all running different programs, and each visible split of the terminal is a pane. More on this will be covered in the later chapters.
Tip
What is a session in tmux?
It may be useful to think of a tmux session as a login on your computer.
You can log on to your computer, which initiates a new session. After you log on by entering your username and password, you arrive at an empty desktop. This is similar to a fresh tmux session. You can run one or more programs in this session, where each program has its own window or windows and each window has its own state.
In most operating systems, there is a way for you to log out, log back in, and arrive back at the same session, with the windows just as you left them. Often, some of the programs that you had opened will continue to run in the background when you log out, even though their windows are no longer visible.
A session in tmux works in much the same way. So, it may be useful to think of tmux as a mini operating system that manages running programs, windows, and more, all within a session.
You can have multiple sessions running at the same time. This is convenient if you want to have a session for each task you might be working on. You might have one for an application you are developing by yourself and another that you could use for pair programming. Alternatively, you might have one to develop an application and one to develop another. This way everything can be neat and clean and separate.