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Fundamentally it's the main protocol that SMS aggregator companies (mostly!) use when communicating with different type of Gateways (GW).
Please check these links...they will definitely help you out understand SMPP more thoroughly. http://www.simpleteam.com/downloads/SMPP_v3_4_Issue1_2.pdf
http://www.ehow.com/facts_7344160_smpp-protocol_.html
Wikipedia definition: "SMPP is a telecommunications industry protocol for exchanging SMS messages between SMS peer entities such as short message service centers and/or External Short Messaging Entities. It is often used to allow third parties (e.g. value-added service providers like news organizations) to submit messages, often in bulk."