Let's see if i can shed some light, as usually wikipedia is not a favored source. While i am not japanese, i have studied plenty of its mythology and folklore and so i like to think i at least know enough to help you grasp an idea if the wiki link had not helped.
To start, the japanese had Spirits, or Kami, that they would revere (or fear,). Generally the word Kami was used to refer to Spirits of natural phenomena; however it would also refer to Gods and other supernatural things. Shrines and offerings were often given to bring good fortune, keep crops coming; simple things of that nature.
A Shikigami was a spirit used by onmyouji; otherwise known as scorcerers or (aha!) "witches". When summoned, they exsisted to protect and serve that onmyouji. They had many forms of different kinds, but were often shown in legend as enchanted japanese paper cut outs. These could put up the illusion or guise of being any form or thing. It was typical for these "paper servants" to carry out tasks like spying or tracking for their masters.
Now, i am unsure how realistic this woud truly be - but i would compare it to the average Astral Familiar if anything.
Hope this helped!