Some plotter's have every single thing planned out from all the twist and turns of the story, to extensive character development. This can run into several pages before one word of the story is written. Other plotters have a less extensive outline, allowing for possible change of direction.
Most mystery writers are plotters by necessity. Twists, turns, red herrings, and bad guys all need to be ready to go.
To a pantser, an outline seems insane. They can't imagine trying to plan how everything will happen. It takes the excitement of discovery away. Just let the character go wherever they want and let the story follow. Of course, this usually means a very rough first draft which must undergo extensive re-vision. That's not to say the plotter doesn't have to revise, but probably a lot less.
Which kind of writer are you? And would you ever give the other way a try?