Wikipedia has the same definitions that are in your textbook, but often provides much more readable proofs as compared to the very short and "elegant" ones or u

Wikipedia has the same definitions that are in your textbook, but often provides much more readable proofs as compared to the very short and "elegant" ones or unwieldy monstrosities in your usual textbooks. At least that's true for textbooks at the level of, say, Dummit&Foot's Abstract Algebra or Baby Rudin.