Called a sapodilla back home, and ciku (chikoo, chiku) in Malaysia (and sapota or zapota in other places), we actually had a hard time getting a ripe version of this. From the outside, it looks rather like a small potato, having a rough brown, grainy skin and oblong appearance. When ripe, they are somewhat soft when squeezed, like a tomato. The inside flesh is soft and and earthy brown in color, it looks and feels almost like a well-cooked sweet potato. Contained within are a few hard black seeds. When unripe, the flesh is hard and extremely drying to the mouth, like an unripe persimmon (or the most heavily over-brewed tea you can think of). When ripe though, the soft flesh is deliciously sweet and malty, almost like a toasted brown sugar or caramel flavor.
For more info, see the wikipedia article: https://en.wikipedia.org/wiki/Manilkara_zapota