The apples that you buy at the market are all from the same species of plant, Malus domestica. Within that species, there are 7,500 different varieties (or cultivars) of apples. The list of apple cultivars includes Red Delicious, Macoun, Honeycrisp, Granny Smith, and the like. They look and taste different but are all recognizable as apples.
Brassica oleracea is a species of plant that, like the apple, has a number of different cultivars. But these cultivars differ widely from each other: cabbage, kale, broccoli, brussels sprouts, kohlrabi, collard greens, and cauliflower. Nutty that all those vegetables come from the same species of plant. (via @ajsheets)