It's called keratosis pilaris, which means little bumps of keratin protein in the hair openings. It affects just the spots you describe, and it's genetic. If you want, you can smooth them out by applying an alpha-hydroxy moisturizer (Am-Lactin is one-they're nonprescription) and exfoliating after a shower daily with a BufPuf or loofah. The bumps of course return when you stop, so people differ in how much effort they want to put into the exercise.
Best.
Dr. Rockoff