Deep packet inspection - Definition from the Hotspot Shield Glossary
DPI involves packet filtering at the Application Layer of the OSI model. Deep Packet Inspection is an advanced method that allows the identification of packets that conventional packet filtering is unable to detect. The packets that are inspected can be classified, rerouted and identified and usually contain specific data or code payloads.
DPI is useful in prioritizing network traffic and improving the flow of high-priority data - The high priority data will be routed first while less important data will be routed afterwards - An example could be video streaming as the important data, and simple internet browsing as secondary data. Although DPI is useful in improving traffic flow, it can also create additional problems and be exploited by hackers; furthermore it can have a negative impact on processing speeds due to the extra burden it adds.