Paypal express checkout is a feature of Paypal and is integrated by default in Magento.
Paypal allows to capture the amount of the order. The Seller has to register on Paypal using an email for a business account. Once approved he can use this email for capturing the order amount from buyers. Once buyer selects the products to purchase, he is redirected to Paypal page for verifying his purchase. Once authenticated, he is redirected back to the review page where he has to place the order to confirm. This tag denotes all the issues related to the paypal's express checkout feature.