Purchase order is the confirmation of delivery of material. 
Invoice is made when material will deliver.

In Purchase order they will specify what are the items to be
purchased

In Invoice they specify what items we have purchased

purchase order comes after Invoice i can say, after 
confirmation of all the price quoted in the invoice we will 
be decide to purchase right.
Invoice is nothing but it is like quote summary of price of 
goods/materials

Invoice is a list of price, rate, quantity, quality and we 
can say that its all about bills of materials which are 
dispatched.
Purchase order is the list of materials  ordered by the 
company to the mfrs.

Puchase order is the document in which Customer will mention
the items to be required for them & also Terms of Delivery.

But Invoice is the document proof made by the supplier, 
stating that they had supplied items to the customer as per 
the purchase order of the customer.