Pantsing (also known as winging it) is the term Wrimos use refer to writing without a fixed outline (an outline that the writer will force themselves to follow). A Wrimo who adopts the pantsing approach to writing is called a pantser.
While Pantsers are united in not doing a fixed outline, they have varying opinions on whether a lesser degree of planning is good. Because of this, pantsers vary greatly in extremity. The Extremist Pantser does absolutely no planning at all, they do not even come up with a vague idea before November 1. Others come up with an idea before November 1, but lack an outline of any sort. Others come up with an outline and/or synopsis before November 1, but the outline/synopsis is not fixed. Still others do little or no pre-November planning, but find time to write a flexible outline during November.
Planners are the opposite of Pantsers.
Pantsers, especially extremist pantsers, are prone to dares, adoptables, and other NaNoWriMo tributes such as Mr. Ian Woon. Typically, it is harder for a pantser to win than it is for a planner to win, and a pantser's week two is often worse than a planner's week two.