Tessellation  
A pattern made of identical shapes.
the shapes must fit together without any gaps,the shapes should not overlap.
REGULAR polygon 
To be a regular polygon all the sides and angles must be the same.  | 
IRREGULAR polygon 
Irregular polygon have different sides 
Then a regular polygon.  
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Ref:  Salzman, L., (1990).  Cool Tessellations.  Creative activities that make math and science fun for kids!  P6-7) 
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 3 Facts: about regular polygons
1.A polygon is a pane shape (two-dimensional) with straight sides. Examples include triangles, quadrilaterals, pentagons, hexagons and so on.
2.Regular polygons tend to look like someone tried to make a circle out of some straight lines. 
3.A polygon is a flat figure that is made up of straight lines and is enclosed.
2 facts: about IRREGULAR polygon
1.Irregular shapes have sides and angles of any length and size.
2.irregular polygon has the same amount of sides.
For more facts about polygons!