Orange liqueur is a liqueur with the predominate flavor of oranges. There are many different kinds of orange liqueur available on the market, but the vast majority of them fall into one of two categories: those that are clear and taste of citrus, and those that are brown and have wood barrel flavors that accent the orange. While there are other orange liqueurs that fall into neither of those categories (I'm looking at you, Compass Box Orangerie!), those are the exception.
While orange liqueurs have probably been made since the advent of distilling, the safest place to start is on the island of Curaçao in the 16th century. Spanish explorers brought their native Valencia oranges to the island to plant groves (citrus plants travelled with explorers so a source of Vitamin C could be established wherever they landed). The soil and climate were improper for the growth of good fruit, and the small, stunted green oranges that developed were inedible, even to the local goats.
Over time, these oranges developed into what is now known as the laraha, the native orange on Curaçao. In the 19th century, the Senior family began to make a liqueur based on the dried peels of the laraha. This liqueur is known as Curaçao and is clear, or can be artificially colored orange, blue, or green.
Triple Sec is a type of Curaçao with about a third of the sugar. Sec means "dry". The first Triple Sec was made in 1834 by Jean-Baptiste Combier, who used citrus peels from Haiti and made a liqueur near Saumur in northeastern France. Triple Sec is clear, and best known brand of Triple Sec is Cointreau, but there are other producers, such as Combier L'Original Liqueur D'Orange, Bols, Hiram Walker, Patron (who makes Citronge), and Luxardo (makers of Triplum).
The other style of orange liqueur is one where there is a flavor profile of wood barrel along with the dominant note of orange. These can either be aged in barrel, or use a wood-aged spirit as the base for extraction. Some common brands that have this flavor profile include Grand Marnier, but there are also Gran Gala, Mathilde X.O, Mandarine Napoleon, and Clement Creole Shrubb.
Some orange liqueurs use Scotch whisky as their base, like Orangerie from Compass Box. There are other modern producers, like Fruit Lab (makers of Citry, from organic oranges), or Paula's Texas Orange that don't quite fall into any category neatly. As friendly warning, inexpensive half gallon bottles of Triple Sec are usually artificially flavored and are cloyingly sweet.