The Apple A6 system-on-a-chip (SoC) was introduced in 2012 alongside the iPhone 5 and it's graphically more powerful version, the Apple A6X was introduced shortly thereafter for the iPad 4. They represent the Apple's first custom processors. Previous versions of Apple's A-series, including the A4, A5, and A5X, all ran existing processor designs like the ARM Cortex A9. This year, instead of sticking with the Cortex A9 or moving on the new ARM A15, Apple licensed the ARM v7s instruction set and rolled something uniquely their own — a 32nm CMOS dual-core Apple CPU that can run from between 800MHz and 1.2GHz. Likewise, instead of going with the dual-core PowerVR SGX543MP2 graphics chip found in the iPhone 4S, or the giant quad-core PowerVR SGX543MP4 found in the iPad 3, Apple went with the triple-core PowerVR SGX543MP3 GPU. And they topped it all off with 1GB of RAM.
Apple A6: Swift processor
Apple's first custom processor was called "Swift". Existing between the ARM Cortex A9 and A15 freed Apple to find the best balance of performance vs. power consumption. What's more, Apple laid out the processor manually. That is to say, by hand. That's almost unheard of these days, and shows just how seriously Apple is taking every bit and atom of the iPhone's design.
Apple sought to make the iPhone 5 twice as fast as the iPhone 4S and they've succeeded brilliantly. The Apple A6 is an absolute beast.
The Apple A6X cranks the swift processor up to 1.4 GHz. It also goes from the giant quad-core PowerVR SGX543MP4 found in the iPad 3 to an updated PowerVR SGX554MP4 for the iPad 4.
The iPad 3 was notorious for getting warm to very warm under heavy load, like playing lots of 3D games. That only grabbed attention because previous iPads had always remained freakishly cool. The A6X's more modern architecture doesn't eliminate the warmth, but nor does the higher clock speed exacerbate it. It just takes the edge off.
Apple A6 and A6X: The bottom line
The Apple A6 was replaced by the Apple A7 in 2013.