A pentagon is a 2-D shape (it is in fact a polygon) that has 5 straight sides. If all of the sides are the same length and all of it's angles are the same size, it will be a regular pentagon.

The perimeter is the total length of the outside of a 2-D shape. If you walked around the outside of a field and finished up where you started from, you would have walked around the field's perimeter. As it is a length, you measure a perimeter in centimetres (cm) and metres (m).

A polygon is a 2-D shape that has straight sides (it must be 3 or more - try it for yourself if you don't believe me!) and doesn't have any gaps leading from it's inside to the outside (it is 'closed', as they say, whoever they are).