Applejack is a distilled liquor made from apple cider. You can think of it as a fruit-based brandy made from apples. It was hugely popular in Colonial America, so much so that it was used as a form of currency in some of the northern colonies (especially in New Jersey).
Apple cider was ubiquitous during this time because the water was so bad people tended to drink cider instead. But cider was heavy to transport and didn't keep all that well. Freeze distillation was used to turn cider into a harder alcohol. Cider left outside during the winter would freeze. The ice was removed leaving the alcohol in a stronger proportion to the remaining water. This was a simpler process than steam distillation and was referred to as "jacking" (hence Applejack's name).
The downside was that the undesirable cogeners and fusel alcohols remained (the stuff that gives you a hangover). These compounds were not removed as they are in steam distillation. So applejack gained a provincial reputation and as steam distillation matured the freeze distilled variety was relegated to the backwoods. Applejack has never recovered it's once massive popularity but it's probably not going away since it is a component of some relatively famous classic cocktails.
Today Laird & Company is the main producer of Applejack. Their entry level product is just called Applejack. It is a blended spirit that contains a proportion of neutral spirits so you are not getting a lot of apple character.
Instead I highly recommend you use Laird's Bonded Straight Apple Brandy which is not blended with neutral spirits. You are getting 100% apple distillate which shows in the aroma and flavor. This product is 100 proof which is closer to the stronger spirits that were the norm a century ago when the best known Applejack cocktails were being invented. The bonded version is only a few dollars more than the straight version and in my opinion worth the price.
Laird's also makes a 7 1/2 year and a 12 year straight apple brandy. I have not been fortunate enough to try these yet. If someone has please let us know what you think of them.