What's the difference between Trigonal Planar and Trigonal Pyramidal?

Ammonia (trigonal pyramidal) Rotate molecule /  Toggle lone pairs on/off

Formaldehyde (trigonal planar) Rotate molecule

From the top, these two molecules both look triangular.  But from the side, you can see that the central nitrogen atom of ammonia projects slightly above the plane of the triangle to form a pyramid.  In formaldehyde, the central carbon is in the plane of the triangle (rotate molecules to see side views).  The differnence is that ammonia has a lone pair which repels the three bonding pairs of electrons to form the pyramid.  Formaldehyde does not have a lone pair so the molecule is flat.