A polyhedron is a three-dimensional geometric shape composed of flat polygonal faces, straight edges, and vertices. The faces can be any polygon, such as triangles, squares, or pentagons, and they are connected in such a way that they enclose a volume. Common examples of polyhedra include cubes, tetrahedra, and octahedra. Polyhedra can be classified into various types, such as regular, irregular, convex, and concave, based on their properties.