If you abhor something, it gives you a feeling of complete hatred. Chances are you abhor that kid who used to torture the frogs in biology class.
Abhor is from Latin abhorrere — "to shrink back in horror." It is the strongest way in English to express hatred, even stronger than loathe. We only use abhor in formal contexts; you might say "I abhor that man," but you would be less likely to say "I abhor spinach" unless you tend to express yourself in highfalutin terms no matter what the occasion.