A tetrahedron is a polyhedron composed of four triangular faces that meet at each vertex. It is one of the five Platonic solids and is the only convex polyhedron with four faces. A regular tetrahedron has all four triangular faces that are equilateral triangles. The net of a tetrahedron is an arrangement of edge-joined triangles that can be folded along edges to form the triangular faces, and a vertex figure is the shape exposed when a corner is sliced off.

1. Tetrahedron In geometry, a tetrahedron is a polyhedron composed of “ four triangular faces” three of which meet at each vertex A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids. The tetrahedron is the only convex polyhedron that has four faces. The tetrahedron is one kind of pyramid.

2. Tetrahedron Net View In geometry the net of a polyhedron is an arrangement of edge-joined polygons in the plane which can be folded (along edges) to become the faces of the polyhedron.

3. Tetrahedron Vertex View In geometry a vertex figure is, broadly speaking, the figure exposed when a corner of a polyhedron or polytope is sliced off.

4. Sources http://en.wikipedia.org/wiki/Tetrahedron