The sweet orange is, for most of us, most of the time, the same thing as an orange. While there are other oranges -- the bitter orange (aka the Seville orange), the mandarin, and the citron -- most Americans will refer to these by their specific names, and when they say orange, they mean a sweet orange.
The sweet orange is believed to be a hybrid of the pomelo and the mandarin, and as such is given not a species name but a hybrid name: while we strongly suspect that it should be C. maxima × C. reticulata, it is commonly referred to as Citrus ×sinensis, as it was named before the hybridization was deduced; our first written record of the sweet orange was in China in 314 BCE.
Which is to say, the sweet orange has been under development for over 2000 years. As you might expect, it currently occurs in a number of cultivars, including the navel orange, the Valencia orange, the Hamlin orange (one of the most popular juice oranges in America), the Midsweet orange (a midseason juice orange), the Parson Brown orange (once the most popular juice orange, it's now losing ground to newer varieties), the Pineapple orange, the Cadanera orange, the Jaffa orange, and many many many more.
There are also many hybrids descended from the sweet orange, including the blood orange, the tangor, the kiyomi, the orangelo, and the grapefruit.
Iron Noder