Data sent over computer networks, such as the Internet, is divided into packets. The computer or other device that receives these packets then reassembles them. So a packet is a small segment of a larger message.
Deep Packet Inspection (DPI), is a type of network packet filtering. In other words, deep packet inspection can find, detect, classify, block, or reroute packets with specific code or data payloads that are not found, found, classified, blocked, or redirected by traditional packet filtering. Deep packet inspection, contrary to plain packet filtering, examines more than just packet headers.