If these terms still sound confusing to you, just take a look at the figure below:
Focal length is the distance from the focusing plane (where the camera’s sensor is) to the rear nodal point of the lens, when focused at infinity. This is a property of the lens, which determines the angle of view as well as the perspective.
Focusing distance is the distance from the focusing plane to the subject.
Minimum focusing distance is the shortest distance from the focusing plane to the subject, at which it can be sharply rendered, i.e. “in focus”. If we move the camera closer to the subject, we won’t be able to focus on it. Usually in lens properties, lens makers indicate the minimum focusing distance, instead of the working distance, which is far more important in practice.
Working distance is the distance from the front element of the lens to the subject.
Effective working distance is the distance from the outer edge of the lens to the subject. This term has a meaning when the lens’ front element resides behind the outer edge of the lens, for example the Sigma 105/2.8 EX Macro and many other macro lenses.
Borislav Kostov - photography and design 