Polygons
An Introduction to Polygons
Shape 
Classification 
Triangle 3 sides Tri means three * There are some special triangles that are important to know: Equilateral: All three sides are the same length and all three angles are equal. Isoceles: Two sides have the same length, the angles opposite the equal sides are equal. Scalene: NO two sides have the same length and no two angles are equal. Acute: All three angles are less than a right angle. Obtuse: One angle is greater than a right angle. Right: One angle is 90 degrees  a right angle. 

Quadrilateral 4 sides Quad means four * There are some special quadrilaterals that are important to know: Parallelogram: the opposite sides are parallel. The angles are also equal. Rectangle: all the angles are right angles. Square: all four sides are equal. Rhombus: all four sides are equal, it looks like a square on a slant. Trapezoid: two sides are parallel and two sides are not parallel. 

Pentagon 5 sides Pent means five 

Hexagon 6 sides Hex means 6 

Octagon 8 sides Oct means 8 

Decagon 10 sides Dec means 10 
These shapes
are only examples, there are several different shapes that can apply to these
polygons.
Related Links:
Measuring
area and perimeter of shapes.
Geometry Tutorials