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Major modes in Emacs control how buffers behave. Each buffer will always have a major mode. You are free to change a buffer’s major mode at any time by typing the command for another one. Each buffer can have just one major mode.
Minor modes, by contrast, are typically optional add-ons that you enable for some (or all) of your buffers.
The major mode is always displayed in the modeline. Some minor modes are also displayed in the modeline, but usually only the ones that alter the buffer or how you interact with it in some way.