Orange juice is the juice squeezed from sweet oranges (as opposed to bitter oranges). The juice is either sold as is (not from concentrate), concentrated for later reconstitution with water, or dried for later rehydration with water.
For your cocktails and mixed drink recipe, you can get away with the first two though I suggest you use a good quality not from concentrate orange juice. By all means you can also squeeze oranges yourself - that is always preferable if your oranges are ripe.
Due to it's worldwide reach, orange juice is used in a lot of drinks. Some of the most common are champagne and orange juice (Mimosa), vodka and orange juice (Screwdriver), and gin and orange juice (usually just called Gin and Juice).
In fact you can mix pretty much any base spirit with orange juice for a highball-style, citrusy drink. Orange juice mixes with almost anything as long as you aren't doing sweet on sweet where you are making an unbalanced cocktail by using orange juice (sweet) with another sweet ingredient (a liqueur for example) without an acidic or bitter ingredient to counteract the sweetness.