The system will launch immediately on Google's new Nexus 5 phone and then roll out to other Nexus devices, the Samsung Galaxy S4 and HTC One, in the next few weeks.
Nexus devices are Google's flagship line of phones and tablets, running the newest and purest form of Android and available without contracts. The Nexus 5, manufactured by LG, is built to work internationally on a variety of bands and with whatever local SIM card you pick up, though Verizon's band is still not supported.
Physically, the 5-inch Nexus is a bit thinner and lighter than the previous generation, while packing a faster Snapdragon 800 processor. It has optical image stabilization and a new HDR camera feature for combining multiple exposures into a single photo, lightening up dark areas and bringing details back to images blown out by a flash.
The Nexus 5 is available online today in 10 countries and will be available in retail stores like Best Buy. It's a wider rollout than for previous generations of Nexus phones. The 16 GB version will cost $349 without a contract and the 32 GB version will go for $399, also without a contract.
The phone also will be the first to have the new Kit Kat features, many of which attempt to break down the walls between web, search, apps and communication tools.
The phone app -- widely neglected since Android smartphones first came out -- gets more search power in Kit Kat, pulling in search results for business as well as your usual roster of contacts. Incoming calls from business are recognized automatically in caller ID, while pulling data from the same database that powers Google Maps.
Kit Kat also adds support for third-party cloud services like Box so you can see files stored on the device and in the cloud. Google Hangouts absorbs SMS and MMS messages, so your chats, video calls and texts are all together in one place.
Google Now, the company's app for serving up answers to questions you haven't even asked yet, is adding new categories. It can deduce your interests based on your web searches -- say, "The Walking Dead," or corgis -- and add a card to Google Now with the latest information about those interests. If there's a specific site you check a lot that is only sporadically updated, Google Now might alert you to new posts when they happen.
It's also tapping the power of crowds. Google Now knows your location, and Google knows what people at that exact spot during that time are most likely to search. For example, someone standing in front of Old Faithful at Yellowstone might search for a schedule of when the geyser is going to erupt. Google Now will automatically show you the times without your searching.
Google Search on Kit Kat is not limited to content from the web. Google is starting to index content deep inside of individual apps, so if you search for a recipe it might show a web page and a link to a cooking app you already have installed. For now, it will only work with installed apps, but Google says its working on adding support for apps in the Google Play store, which could be a clever way to sell more apps.
Kit Kat is also Google's attempt to address one the bigger problems plaguing Android: OS fragmentation. Low-end Android devices are big sellers in emerging markets like China and India, but because of their limited memories and pokey processors, they typically run older versions of the Android operating system, like Gingerbread.
The decision of what version of Android to ship on these devices is up the manufacturers. But Google is trying hard to make its newest OS more appealing for these markets by upgrading it to use less processor power and memory.