A Closed shape with any number of straight side is referred to as Polygon
Or We can say that:
A Simple Closed curve made up of only line segment is known as Polygon
Below are commonly known Polygons :
Type of Polygon  Number of side or Vertices  Rough Figure 

Triangle  3   Quadrilateral  4   Pentagon  5   Hexagon  6   
Septagon or Heptagon  7   Octagon  8  
All these types of polygons are discussed in detail in the above respective links

