Siri is a virtual assistant with a voice-controlled natural language interface that uses sequential inference and contextual awareness to help perform personal tasks for iOS users. Spun out of the DARPA-funded CALO project, Siri was initially developed by Dag Kittlaus and his team of SRI International as an iPhone app. Apple bought Siri in 2010. In October of 2011 Apple re-released it as part of iOS 5.
Siri in iOS 5
Siri was re-introduced with a new interface as part of iOS 5. Although it no longer enjoyed the same level of partner integration it had as an independent app, Apple's Siri better integration with built-in services like iMessage, Mail, Calendars, Reminders, Notes, Stocks, Weather, Music, Safari, and a few external services like Google and Wolfram Alpha. It could also tie into Find my Friends.
Siri in iOS 6
With iOS 6 Siri made compatible with the iPad, and gained the ability to provide information about sports, movies, and restaurants, as well the ability to open apps, post Facebook or Twitter updates, and hook into Apple's new Maps app for turn-by-turn navigation.
Siri in iOS 7
With iOS 7 Siri got a facelift to match the new design language, new, higher-quality voices, access to Settings and Voice Mail, the ability to display tweets from Twitter, tie ins to Wikipedia and Bing,
Siri in iOS 7.1 and CarPlay
With iOS 7.1 Siri was integrated into the new CarPlay feature. Siri also gained higher quality voices for Mandarin Chinese, U.K. English, Australian English, and Japanese, and a new push-to-talk feature to better control when Siri starts listening, and when it stops.
Siri in iOS 8 and beyond
While Siri itself is integrated into the iOS Springboard system, the Siri team is still run quasi-independantly at Apple. Arguably modularizing Siri and integrating the team could allow for a broader scope of services over time. Apple has not yet released Siri for OS X on the Mac, nor have they provided an application programming interface (API) for developers to hook into. (Though it's unclear how that might affect partnership deals, and how collisions could be handled.) Also, unlike Google Now, Apple hasn't yet put Siri on-board iOS devices, meaning even local tasks like setting alarms have to go out to the internet, allowing the network to affect performance. Likewise, Apple has thus far refrained from adding prescience to Siri, which would allow it to provide information before being asked.
Siri is rumored to be gaining even more new features in iOS 8.