A Purchase Order is analogous to a Sales Order. It is usually a binding contract with your Supplier that you promise to buy this set of Items under the given conditions.
In ERPNext, you can make a Purchase Order by going to:
Buying > Purchase Order > New Purchase Order  A Purchase Order can also be automatically created from a Purchase Request or Supplier Quotation.
Entering a Purchase Order is very similar to a Purchase Request, additionally you will have to set:
If your Supplier is going to charge you additional taxes or charge like a shipping or insurance charge, you can add it here. It will help you to accurately track your costs. Also if some of these charges add to the value of the product you will have to mention in the Taxes table. You can also you templates for your taxes. For more information on setting up your taxes see the Purchase Taxes and Charges Master.
Many times, the tax paid by you to a Supplier for an Item is the same tax you collect from your Customer. In many regions, what you pay to your government is only the difference between what you collect from your Customer and pay to your Supplier. This is called Value Added Tax (VAT).
For example you buy Items worth X and sell them for 1.3X. So your Customer pays 1.3 times the tax you pay your Supplier. Since you have already paid tax to your Supplier for X, what your owe your government is only the tax on 0.3X.
This is very easy to track in ERPNext since each tax head is also an Account. Ideally you must create two Accounts for each type of VAT you pay and collect, “Purchase VAT-X” (asset) and “Sales VAT-X” (liability), or something to that effect. Please contact your accountant if you need more help or post a query on our forums!