A combat order is a set of instructions that you receive from a superior, and delegate to subordinates. As a leader, it is up to you to select the information from "higher" that you need to delegate down, along with any coordinating instructions and tasks you need your subordinates to know and execute.
At OCS you will take part in squad and fire team tactical exercises. You will receive orders from higher, your instructors, and then create your own order and explain it to your peers. The Combat Order has three versions, the "5 Paragraph Order," also called the "Operation Order", "Warning Order," and "Fragmentary Order."
The 5 Paragraph Order is the full version of the order. You receive one from higher, then you create your own.
The Warning Order only has some sections of the order, and is posted in a physical location, like a wall or a tree, that lets your troop know some quick details and that a 5 Paragraph Order will be briefed some time in the near future.
The Fragmentary Order is an update to the 5 Paragraph Order. It only contains the relevant, updated sections.