elaborate your question

it depends on how you define it and what Kind of structure you are analyzing. In the thin-walled cylindrical world, the buckling load is defined as the Maximum load in the load-shortening curve. The critical load is usually calculated by a theoretical formula or linear buckling Analysis.

Regina is right. Critical load is strictly the load at which the stability based upon the quadratic form of the total potential energy changes from stable to unstable and is often associated with a linearised eigenvalue analysis. However, such a critical state can be either stable or unstable depending upon the nature of the higher energy terms in the expansion of the total potential energy (tpe) about the critical equilibrium state. The critical load for a column for example is stable in that the fourth order terms in the expansion of the tpe would correspond with an increasing energy and the post-critical equilibrium path passing through the critical state would relate to a stable symmetric bifurcation. Most shells on the other hand have the higher energy terms negative and relate to a falling post critical equilibrium path - often of an unstable symmetric post critical behaviour.     

Buckling on the other hand can usefully be defined as the load at which a dynamic change in equilibrium state will occur. For a force controlled loading system the maximum load usually associated with the behaviour of an imperfect system will therefore also correspond with a state of critical stability and so is also a critical equilibrium state. For a displacement controlled loading system on the other hand the buckling load will generally not correspond with the maximum load but will still be a state of critical and unstable equilibrium. In this sense a buckling load will always relate to a critical load but a critical load may not be a buckling load.

Sorry to go on but I just happened to come across this discussion; this is an issue that I have for longer than I care to recall been keen to see clarified in the literature since the terms are used very loosely and cause confusion.    

Critical load is that obtained mathematically corresponding to the neutral equilibrium. This is calculated for an ideal structure which never exists in practice. It belongs to an ideal situation. Here the prebuckling deformation is neglected.

As far as real structure is concerned, there will be deformation from the beginning load is applied due to inherent imperfection in geometry and/or eccentricity in loading. So It becomes a beamcolumn problem. The maximum load reached is called the buckling load or limit load. Here the prebuckling deformation is duly accounted for by drawing the complete load deflection response.