There are lots of questions floating around regarding ART — the new experimental runtime that's included as a developer option in Android 4.4 KitKat. Anyone can enable it — just go to the developer settings.
ART is a new Android runtime being introduced experimentally in the 4.4 release. This is a preview of work in progress in KitKat that can be turned on in Settings > developer options. This is available for the purpose of obtaining early developer and partner feedback.
Will it make a difference to you as an end user? Maybe yes, maybe no — or maybe placebo? The short, short version? It's way early on this right now and most of us don't need to worry about it quite yet.
We talked some more about it on this week's podcast, and it's worth a five-minute breakout.