Polyhedrons
A polyhedron is a solid with flat faces
(from Greek poly meaning "many" and hedron meaning "face").
Each face is a polygon (a flat shape with straight sides).
Examples of Polyhedra:
Its faces are all squares
Its faces are triangles
and rectangles
What faces does it have?
No curved surfaces: cones, spheres and cylinders are not polyhedrons.
Common Polyhedra
Cubes and Cuboids (Volume of a Cuboid) 

Platonic Solids  
Prisms  
Pyramids 
Note: the plural of polyhedron is either polyhedrons or polyhedra
Many More
Explore 100s of Animated Polyhedron Models. You can also see some Images of Polyhedra if you want. 
Counting Faces, Vertices and Edges
When we count the number of faces (the flat surfaces), vertices (corner points), and edges of a polyhedron we discover an interesting thing:
The number of faces
plus the number of vertices
minus the number of edges equals 2
This can be written neatly as a little equation:
F + V − E = 2
It is known as Euler's Formula (or the "Polyhedral Formula") and is very useful to make sure we have counted correctly!
Example: Cube
A cube has:
 6 Faces
 8 Vertices (corner points)
 12 Edges
F + V − E = 6 + 8 − 12 = 2
Example: Triangular Prism
This prism has:
 5 Faces
 6 Vertices (corner points)
 9 Edges
F + V − E = 5 + 6 − 9 = 2
But there are cases where it does not work! Read Euler's Formula for more.
Diagonals
A diagonal is a straight line inside a shape that goes from one corner to another (but not an edge).
A polyhedron can have lots of diagonals. Can you think of one without diagonals?