The color triangles formed by velocity vectors u,c and w are called velocity triangles and are helpful in explaining how pumps work.
: and  are the absolute velocities of the fluid at the inlet and outlet respectively.
: and  are the relative velocities of the fluid with respect to the blade at the inlet and outlet respectively.
: and  are the velocities of the blade at the inlet and outlet respectively.
: is angular velocity. 
Figures a and b show impellers with backward and forward-curved vanes respectively.