The 30-pin dock connector first appeared on the iPod 3G in 2003, and has been on all iPods, iPhones and iPads ever since, with the sole exception of the first generation iPod Shuffle. The first gen Shuffle looks like a thumbdrive (or what some would call a pen drive) and used a USB male connector, whereas the first and second generation iPods had a FireWire port at the top.
iPod Accessory Protocol
Since its introduction, Apple made several minor modifications to the electronics interface of the dock connector, but the physical connector itself remains unchanged. Collectively, this and the communications protocol is called the iPod Accessory Protocol (iAP).
Initially, they introduced different resistor values on the “accessory detect” pin when they allowed third-party companies to make docks and car adapters. At that time, the dock connector mainly had audio in and line out functionality (connected to the back of the docks), as well as a serial interface for remote control via the dock. The serial protocol was largely reverse-engineered by maushammer (website no longer accessible) and this guy here (I think he’s called Christoph but it’s not on that page). This was also used by car manufacturers to allow iPod playback control from buttons on the steering wheel.
When Apple released the iPod Video that was capable of playing videos in 2005, they added video out (composite and S-video), as well as an authentication chip to allow only authorized docks and cables to receive video out (including audio). Soon enough, China caught up with their release of “authorized” accessories, which contain the authentication chip that can be re-purposed for other use.
Presumably, that was also when they added USB support for iAP, which I’m pretty sure also requires an authentication chip. USB support would allow a host to communicate and control the iPod through a USB cable. My car stereo correctly recognizes the iPhone as an iPod over a USB cable.
The iPod Camera Connector was also introduced for the iPod Photo and iPod Video in 2005. This was a small “dongle” that has a dock connector on one end and a USB port on the other. Oddly, according to an Everymac article, later iPods released in 2006 do not support this accessory any more. It is unknown if they somehow switched the USB interface to host mode, or if they used a separate chip to emulate this.
In 2008, charging via FireWire was no longer supported with the introduction of the 2nd generation iPod Touch and 4th generation iPod Nano. The pins dedicated to FireWire in the dock connector are now unused.