The difference is the receipt on Voucher is a quittance not an invoice.

Example: Sale Receipt -> You go to a shop and pay directly. The Shop Owner give you a Sale Voucher. This Voucher is capture as a Sale Receipt. Sale Invoice -> You order a kitchen and get a Invoice, which you will pay when you get the kitchen. This Invoice is capture as a Sale Invoice.

You can read this in the OpenERP documentation.

"When you sell products to a customer, you can give him a true invoice or a Sales Receipt, which is also called Customer Receipt. Sales Receipts are merely a kind of sales ticket and not a real invoice.

When the sales receipt is confirmed, OpenERP creates journal items automatically and you can record the customer payment related to this sales receipt. The easy invoicing system does not require extensive accounting setup, so you will be up and running quickly!"

You can read this on this website: doc.openerp.com/6.1/book/3/3_6/invoice_payment/ "Under Simple Customer Receipts" The documentation is for Version 6.1 but this is also valid for the oterh Versions.

In my "real world" experience, while Invoices always record the full amount of a "trade", Receipts travel in the reverse direction of money, regardless of whether the value is the full, or partial, payment of a trade. As distinct acts, for distinct purposes, a distinct record is required.

I expect, but can't yet confirm (I'm learning too) that the "OpenERP world" follows the "real world" in such things.

I have assigned the correct VAT code to all of the purchase receipts I just entered but they are not being rolled on to the VAT return details . Why ?

The measure difference is : In Supplier Invoice you can record product wise invoice, and when you want to record direct without product so you can use purchase receipt. And same condition with Customer Invoice when you want to record your sales product wise so you can use Customer invoice option, but when you want to record your sales without product so you can record Sales Receipt option.

Invoice is a document that a vendor issue to the purchaser as a request for the payment. Invoice is issue before the payment.

Whereas, receipt is the document issued by the vendor to the purchaser after the the payment.

Invoice: Document issued before Payment

Receipt: Document issued after the Payment.

From my experience in the paper converting field , a receipt is given in shopping outlets or restaurants . An invoice usually is issued in corporate offices either B2B or B2C . 

Invoices are issued prior to the customer sending the payment, whereas a receipt is issued after the payment has been received. The invoice acts as a request for payment, and the receipt acts as a proof of payment. This also means each document requires different information.