A polygon is a simple closed curve made only of line segments. Triangles, quadrilaterals and pentagons are polygons; a circle, an open curve and a curve with a curved side are not polygons.
Taking one exterior angle at each vertex while walking around the polygon makes one complete turn.
$360^\circ$.
A regular hexagon has 6 equal exterior angles. Each is $360^\circ/6=60^\circ$.
$60^\circ$.
Each exterior angle is $360^\circ/8=45^\circ$, so each interior angle is $180^\circ-45^\circ=135^\circ$.
$135^\circ$.
Opposite sides are parallel and equal, opposite angles are equal, adjacent angles are supplementary, and diagonals bisect each other.
A rectangle is a parallelogram with four right angles. Its opposite sides are equal and parallel, and its diagonals are equal and bisect each other.
A rhombus is a parallelogram with all sides equal. Its diagonals bisect each other at right angles and bisect the vertex angles.
A square has all sides equal and all angles $90^\circ$. Its diagonals are equal, perpendicular bisectors of each other, and bisect the angles.