The Valencia orange is one of the most popular cultivars of sweet orange, and is the classic, prototypical orange for most Americans. It has recently been overtaken in popularity by the navel orange, not because of taste but because people prefer their oranges not to have seeds.
The Valencia orange was imported from the Azores region of Portugal to North America in the 1870s. It was originally considered a fairly standard cultivar of the common orange1. Growers across the USA and Mexico started planting them, but it was William Wolfskill, a Mexican fruit magnate,2 who coined their modern name. He named them after Valencia Spain, which was known for having particularly sweet oranges,2 and California ended up as the king of citrus. Had Florida been quicker to dominate the orange market, we might now call these 'brown oranges' (the original name on the East coast), or 'Hart's Tardiff', oft shortened to simply 'Hart'.
The Valencia orange is smaller than many oranges, produces many seeds when grown in warmer climates, has a comparatively thin, tough skin, and has a tendency to "regreen" -- turn green again after it ripens -- all of which tend to make it a less consumer-friendly orange. It tastes darn good, though, and it has the benefit that it contains very little limonin in its flesh, and thus its juice does not turn sour when exposed to air, a factor that helped orange juice become a commodity separate from the whole orange. Valencias are commonly exposed to ethylene gas if they are being sold in the supermarket, to cause them to lose an outer layer of chlorophyll and erase their ripe greenness; historically this same effect was created by dying them orange.
While the Valencia is still a popular orange, it is now most common in the production of orange juice. However, even in this area it is losing ground, as producers can make more money by choosing oranges that fruit at different times of the year and in different climates, resulting in a more consistent supply of juice. This is, ironically, a refining of the same process that made oranges such a popular fruit in America; the winter-ripening navel in conjunction with the summer-ripening Valencia, plus improvements in shipping and refrigeration, are what originally made oranges a year-round staple on the supermarket shelves.
Footnotes:
1. AKA the 'round orange' in the US, or the 'blond orange' in Europe).
2. His lands have since been subsumed into America, and so he is sometimes described as a Californian fruit grower.
3. The American town of the same name was named after the orange, not vice versa.
Sources
American Heritage: The Social History Of A Singular Fruit
NewCROP at Perdue University: Orange
Wikipedia: Valencia Orange