Google and Oracle have been tied up in a fight for a while over how Java works in the Android operating system. In 2012, Oracle specifically targeted the Dalvik virtual machine that runs on Android devices in its legal assault against Google, though Google eventually prevailed in that case. Google has been working toward replacing Dalvik in secret and users running Android 4.4 will be able to use Google's new alternative.
Android 4.4 will reportedly allow users to select which runtime they use. The available options will be Dalvik and Google's new runtime called ART. Google has stated on its website for developers that ART is a work in progress and is considered experimental at this time. Users wanting to give it a try can enable the new runtime in Android 4.4 via the Settings, Developer Options menu.
Indications are that users won't notice much difference between the two available runtimes for now. ART is said to handle app execution in a fundamentally different way than Dalvik. While Dalvik uses a Just-In-Time compiler and a lot of the compiling is actually done on the user's device each time the app is run, ART is different.
ART precompiles bytecode into machine language when the apps are first installed turning them into native applications. The process is called Ahead-Of-Time compilation. This means that start times for applications can be cut down significantly and execution can be faster compared to using Dalvik. Indications are that Google has been working on ART the last two years. No timeframe for ART to replace Dalvik has been announced.