Dihedral: It is an angle formed by the wings which is upward and horizontal of the wing of a fixed-wing aircraft.

Anhedral: It is an angle formed by the wings which is downward and horizontal of the wing.

Dihedral angle has a stabilizing effect on an aircraft. Increasing the dihedral angle of an aircraft increases the dihedral effect. Dihedral effect is a rolling moment which is a resulting from a non-zero angle of sideslip.

Dihedral angle and dihedral effect have usage in aircraft stability analysis, stability, and wing clearance between the wingtip and runway. 

Anhedral wing can be found on high mounted wings to cancel out some of the dihedral effect, for more easily maneuvering of an aircraft.