The tetrahedron is the simplest of all polyhedra—solids bounded by polygons. It has four triangular faces, four vertices, and six edges. If each edge has the same length and each face is an equilateral triangle, the result is a regular tetrahedron—one of the Platonic solids.
Another group of tetrahedra that some people consider special consists of those that have integer edge lengths, face areas, and volumes.