Well not exclusively Mexican, most latin-american countries have their own version which of course is "the best". Might not sound that good, but it's addictive. Essentially, you soak the fish chunks in an acid, usually lime juice. Along with that you add other flavors like onion, hot chile pepper (habanero and jalapeno for me), cilantro, cellery, tomato. Again, recipes vary and their arent' rules. Usually eaten with crackers or chips of some sort. In places like Panama you can buy this stuff in little stores in styrafoam cups to pick up along with your beers.
Is ceviche like Mexican sushi?