What is a Taskbar in a Computer?
The taskbar is a horizontal bar typically at the bottom of the screen where tasks (applications currently running) can be found. The taskbar also features a button to open the start menu, called the "Start button." Nowadays, they also have the ability to display the desktop if you click on the bottom-right of the taskbar.
The tasks in a taskbar are applications currently running. More specifically, they indicate that a program has opened a window. Programs that don't open windows may be running without appearing on the taskbar. The same is true for programs that can open windows, but don't have any open currently. In general, an application closes when all of its windows are closed, but there are some applications that don't stop running after their last window is closed. In this case, it's often possible to reopen an application's window by clicking on its icon the system tray also located in the taskbar.
Some taskbars have the ability to "pin" applications so they can be launched from the taskbar. For example, the File Explorer comes pinned by default in Windows 11.
"Task bar" Spelling
The term taskbar is sometimes spelled "task bar," however the "taskbar" spelling is used most of the time1.