Zucchini are picked while they're still small. Leave them to grow some more and you will get marrows, which are basically just large zucchini. They don't have as much flavor though, which is why marrows are usually stuffed. The reason for this is their larger size means they contain more water than zucchini, which is why it's best to slice marrows and salt the slices. This draws out the excess water so you can then rinse the slices, and is the same as you'd do with eggplant. In South Africa, zucchini are known as baby marrows.