Watermelons, botanically called Citrullus lanatus belong to the family of Cucurbitaceae. They are thought to be native of South Africa, in particular the Kalahari Desert, where several acient varieties (such as the Tsamma melon) are still present today.
The flowering watermelon plant produces a fruit called by botanists "pepo", characterized by a hard rind and a fleshy pulp: this fruit is derived from an inferior ovary and is characteristic of the Cucurbitaceae. Fresh Watermelon Fresh Watermelon
Watermelons are largely made of water (more than 90%), and that's the likely reason for their name: they have been used as an easily transportable source of water for centuries in ancient history, and Asian cultures tell of several legends about watermelons.
Today, there are several hundred different cultivars, mostly due to the different needs of regional markets: genetic manipulation has allowed for the cultivation of giant watermelons (the largest weighed in at approximately 262 pounds), as well as seedless varieties (derived from cross-pollinating a tetraploid plant with a diploid variety, resulting in a triploid plant with much fewer seeds than normal watermelons).
But seeds aren't necessarily annoying: in some nations of Asia, especially China, roasted seeds are very common and eaten as a snack! Other regions of Africa press them to produce watermelon seed oil, which is common in soups such as egusi. In fact, even the rind is sometimes pickled, or even stir-friend, which means the whole watermelon fruit is edible.