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Cats have anal glands that are located at 10 and 2 beneath the tail (like a clock). These glands secrete an oily, smelly substance and it is expelled every time that the cat poops. This is normal. The only time it's a problem is when the glands become inflamed, infected or otherwise impacted. You can tell when this happens because the cat will lick his bottom, drag it on the floor or try some other way to express the glands and get some relief.
Your vet or groomer can express the glands for you and solve the problem.
Let me know if you still have questions about this.