The Jack-O-Man is a NPC Trader that spawns anywhere in the Overworld at night. It has 1000 Health, and will not be provoked if attacked. You can trade with him to get various types of armor, as well as some other equipment. The Jack-O-Man can also open and close wooden doors. Entering the Jack-O-Man trade screen gets you the Achievement "Halloween Spirit".
Finding the Jack-O-Man
The Jack-O-Man is not a particularly difficult mob to find. They generally spawn out in the open, and finding them should not be difficult, as their orange pumpkin heads stand out in the night. However, the Jack-O-Man also tends to spawn in cave systems, especially ravines, where the player may encounter small groups of Jack-O-Men.