According to the company the "S" in "iPhone 3GS" stands for Speed, no doubt because of the mainly internal changes regarding speed. The iOS device has performance more than twice as fast as the predecessor. Apple has also improved the iPhone camera, increasing its resolution from 2MP to 3MP, and adding video capture and auto-focus function.
Apple iPhone 3GS boasts a digital compass, voice control feature that allows for the controlling of the smartphone and music features of the device by voice, and a better battery with up to 9 hours of web surfing or 5 hours of talk time on 3G. The iPhone 3GS is powered by the Samsung APL0298C05 chip. This system-on-a-chip is composed of an ARM Cortex-A8 CPU core underclocked to 600 MHz (from 833 MHz), integrated with a PowerVR SGX 535 GPU. For more specific info about Apple iPhone 3GS, there is a table of specification with more detailed information on the Apple iPhone 3GS Specs.