The Name-Value Pair (NVP) API provides parameter-based association between the request and response fields of a message and their values. The request message is sent from your website by the API, and a response message is returned by PayPal using a client-server model in which your site is a client of the PayPal server. NVP describes the format of the request and response messages that you send to and receive from the PayPal API. An example of a name-value pair is <FieldName>=<Value>, where <FieldName> is the name of the PayPal API field and <Value> is the information that you pass to or receive from the PayPal API, such as, VERSION=200.