3-4 vs 4-3 And How Are We Able To Switch Back And Forth?

Can one of the more savy football posters on here explain the key differences from a philosophical standpoint regarding 3-4 vs the 4-3 and how it is that we seem to have the personnel to successfully switch back and forth with it?

From my understanding the 3-4 usually requires slightly smaller more athletic LBs with a larger field of responsibility than the 4-3, IIRC.

But I would appreciate anyone's input on this.