A triangular pyramid (tetrahedron) has a triangular base, but a square pyramid has a square base. Both pyramids consist of the vertices of the base joined to one point at the top. The triangular pyramid has 3 such joins, but the square pyramid has 4. The side faces of both types of pyramid consist of triangles, the triangular pyramid has 4 triangular faces in all, and the square pyramid has 4 triangular faces and one square face (base).