What's the difference between Ki and IC50?
In biochemistry the Ki is the dissociation constant of a complex and molecules bound to it. It is a measure for the  functional strength of the inhibitor. This can for example be an enzyme and its substrate, the Ki defines the stability of the complex.

The IC50 on the other hand is the halfmaximal inhibitory concentration of a substance on a biochemical process. This can be an inhibitor of an enzyme. 
See this figure (from the Wikipedia article on the IC50):



The Ki plays an important role in the calculation of the IC50, as it defines the strength of the enzyme-inhibitor complex.