Whats the difference between tetrahedron and a triangular pyramid?

There is no difference. A tetrahedron is a triangular pyramid. You, I figure, are aware of the characteristics of polygons and solids, and know that pyramidal bases can be any polygon and that the pyramid is then named based on that, right? But, the triangular pyramid is the polyhedron that is also called a tetrahedron. "The" tetrahedron, a regular tetrahedron, is made from equilateral/regular triangles.