What is the difference between 'em and them?

I saw my friend writing, 


  Lets Kick'em. 


But I don't know what it means and if it is correct to use. Could you help me?  


Question: What technique/sensor can I use to recognized tagged objects thrown onto a surface?
Answer: A better solution might be to use printed tags and image recognition. There are open source solutions for this - for instance, trackmate. All you will need is a printer, a camera, and a transparent (or possibly frosted) surface.