Traditional brandy is made from grapes/wine, but when you make it from apples/cider, it becomes applejack. Originally, the term was probably used for "freeze distilled" apple brandy, but now days any apple brandy (at least those made in the US) can be called applejack. Normandy France, is famous for their Calvados, which is a well known form of apple brandy.

In the US, the most popular applejack comes from the Laird's company. Their most common form is a blended apple brandy which is made from 35% apple brandy and 65% neutral spirits and bottled at 80 proof. They also make a "Bottled In Bond" version which is straight apple brandy and bottled at 100 proof.

The following recipes on this site use applejack:

Norwegian Wood
aquavit, applejack, sweet vermouth, Chartreuse,
Harvest Moon
applejack, lime juice, orgeat,
Corpse Reviver #1
brandy, sweet vermouth, applejack,
Jack Rose
applejack, lime juice, grenadine,
Pink Lady
gin, applejack, lemon juice, grenadine, egg white,