Push vs. Pull System

From a customer's perspective, it describes how they get an item from upstream in a process flow. In a push system, the upstream processes pushes items downstream without balancing to the customer's demand. In a pull system, the downstream process pulls items from upstream as they need it. A pull system is generally considered to be more efficient by eliminating overproduction upstream and excess inventory downstream.
Related StatStuff Video: 
   -Section 2-Lean, Lesson #2 - System Flow Methods