Generally, shapes made from straight lines are named a something-gon, with the prefix (bit at the front) telling you about the number of sides or angles it has. The general name for any closed shape made from straight lines is a polygon.
The word pent means "five", so a pentagon has five sides.
The word hex means "six", so a hexagon has six sides.
However, we do have special names for 3-sided-gons (triangles) and 4-sided-gons (quadrilaterals) - probably because they are the most commonly used shapes.
A regular polygon has equal sides and equal angles. All the polygons shown in the picture are irregular - they have different side lengths and angles.
- Maths is Fun - notes on the definition of a polygon, including concave and complex polygons.