Door gods are deities whose images are often placed upon each side of an entry to a temple, home or business to protect it from evil spirits. This custom began during the Tang Dynasty, started by the Emperor whom got his two most loyal generals during a night due to a troublesome ghost, and after resting well hung their images on either side of his bedroom door.
There are a number of door gods, but the two most known are Qin Qiong, or Qin Shubao, who is pale skinned and carries swords, and Yuchi Gong, or Yuchi Jingde, has dark skin and typically carrying batons. Another two door gods are immortals ordered by the Jade Emperor to guard peach trees that demons were gnawing at, they are known as Shen Shu (who wields a battle axe) and Yu Lei (who wields a mace).
There are two types of door gods, martial and literary. Martial doors gods are usually generals depicted in battle armour and wielding weapons, often their depictions are life sized. Literary door gods are the deities whom are often imaged by the doors facing inwards to the area to help with feeling peace in ones surroundings.