iPhone was originally announced on January 9, 2007, and released on June 29, 2007. The first iPhone had no support for native third-party software, but Apple encouraged developers to write web-based applications that took advantage of iPhone features such as the touch screen. The iPhone Developer Program, which allows developers to submit their own applications to the iOS App Store for download or sale, was not announced until the release of the iPhone 3G/iPhone OS 2.2.1 in 2008.
In addition to the App Store, iPhone 3G added 3G wireless capabilities (hence its moniker), GPS, a redesigned body, a faster processor and more RAM.
iPhone 3GS started the now-entrenched trend of having a bi-yearly release cycle. In even-numbered years there is a major release, which, while of course upgrading internal components of the device, usually focuses on major external changes(such as iPhone 3G's redesigned body). In odd-numbered years, an "S" variant appears that focuses on internal improvements. iPhone 3GS added a 3 megapixel camera, up from 2 megapixels, which added the capability of shooting video. Voice Control was added and the RAM and processor were also bumped up.
Arguably the biggest new hardware feature of iPhone 4, released in 2010, was the Retina Display, a 316ppi screen with eye-popping color. The internals were upgraded to include an A4 processor, a front facing .3 megapixel camera and a back facing 5.0 megapixel camera. For the first time, a CDMA variant was released in February 2011 for Verizon Wireless, who became only the second US carrier to sell iPhone, but due to their network structure was unable to utilize the GSM model in use on AT&T's network.
iPhone 4S was released in October 2011, debuting a new voice-driven assistant called Siri. It also included a new A5 processor, an 8.0 megapixel back-facing camera that was aimed squarely at the point-and-shoot camera market, and enhanced cellular capabilities that greatly increased download speeds on some carriers. The introduction of 4S marked the first time Sprint carried iPhone.
iPhone 5 was released on September 21, 2012. It is slimmer, lighter with 4-inch 16:9 screen. Although the screen is technically higher-res, it is of the same ppi (316). The phone also includes a custom-designed ARMv7 processor called the Apple A6.
Continuing the "S" trend in odd-numbered years, iPhone 5S came to market on September 20, 2013. Major new features included the A7 system-on-a-chip, which was the first 64-bit processor used in a smartphone; the M7 motion co-processor, a low-power chip which tracked data such as steps taken without the power usage that would be incurred by having the main CPU running; and Touch ID, a revised home button that read users' fingerprints to unlock the device and allow app purchases.
At the same time, Apple took the internal components of 2012's iPhone 5 and put them into a new candy-colored plastic shell, releasing it alongside iPhone 5S as the iPhone 5C. It was priced lower and appeared to be Apple's shot at the mid-tier smartphone market, which had previously been limited to simply re-pricing prior years' models.
After years of speculation that Apple would increase the size of their phones, iPhone 6 and iPhone 6+ were released in October 2014. The devices grew to 4.7" and 5.5" screens, respectively. Major refinements were made to the cameras in both devices, with 6+ gaining optical image stabilization due to having additional room for the camera sensor to physically move within the case. The A8 processor powers both devices.