SPI Communication...

As per Wikipedia... (click link for extensive explanation)

"The Serial Peripheral Interface Bus or SPI (often pronounced "es-pē-ī" [IPA: ɛs pi: 'aɪ] or "spy" [IPA: spaɪ]) bus is a synchronous serial data link standard named by Motorola that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame. Multiple slave devices are allowed with individual slave select (chip select) lines. Sometimes SPI is called a "four wire" serial bus, contrasting with three, two, and one wire serial buses."

Another good introduction to SPI (PDF) by Microchip...

We are dwelling into USB, SPI and UART because we ourselves are learning on how to design and build a system comprising of various sensors reporting/logging data to a central host. We'll be using Hope RF Transceiver Modules for wireless communication and the central host will be communicating with a PC via USB interface... A good write up explaining the transceiver module is here.