According to Stereophile:
The classes each cover a wide range of performance. Carefully read our descriptions here, the original reviews, and (heaven forbid) reviews in other magazines to put together a short list of components to choose from. Evaluate your room, your source material and front-end(s), your speakers, and your tastes. With luck, you may come up with a selection to audition at your favorite dealer(s). "Recommended Components" will not tell you what to buy any more than Consumer Reports would presume to tell you whom to marry! Heaven forbid.
Class A
Best attainable sound for a component of its kind, almost without practical considerations; "the least musical compromise." A Class A system is one for which, with the best recordings, you don't have to make a leap of faith to believe that you're hearing the real thing. With Super Audio CD, DVD-Audio, and Hi-Rez PCM and DSD files now available, we have created a Class, A+, for the best performance in those digital categories. Class A now represents the best that can be obtained from the conventional 16/44.1 CD medium. We also created an A+ class for the turntable, phono preamp, and headphones categories.
Class B
The next best thing to the very best sound reproduction; Class B components generally cost substantially less than those in Class A, but most Class B components are still quite expensive.
Class C
Somewhat lower-fi sound but far more musically natural than average home-component high fidelity; products in this class are of high quality but affordable.
Class D
Satisfying musical sound, but these components are either of significantly lower fidelity than the best available or exhibit major compromises in performance; limited dynamic range, for example. Appearance in Class D still means that we recommend this product. It's possible to put together a musically satisfying system exclusively from Class D components.
Class E
Applying to "Loudspeakers," these are entry-level products.