Puppies! Mammillaria are generally sold as a small 2-6 inch plant. They are easy to grow. Flowers are arranged in a circle around the top of the cactus. They need some shade in hot desert areas. Water regularly during growing season and keep dry during winter to induce blooms in the spring.
This particular plant has suffered some drought damage over a couple of years. This has induced it to flower and to make pups this year. The indents are smaller growth periods where the this cactus did not get enough water during grow season. This plant is 30+ years old. It began its journey as a little 2 inch tall cactus that my grandfather gave to my grandmother as a gift.