Apple and Google have quite different goals for what they want in their browsers, and because of that, the Webkit project has become too complex, and too slow, with two big companies pulling in different directions. To increase competition between rendering engines, and to be able to use a much leaner and faster codebase for Chrome, Google has started forking Webkit, into a new rendering engine called Blink.
Webkit is a major rendering engine on the PC, made popular in the recent years mainly thanks to Chrome. There are still two other main competitors to it, though: Mozilla’s Gecko and Microsoft’s Trident. Opera had Presto, but they will be using Chromium’s rendering engine from now on (which is Webkit now, and Blink later).
However, in the mobile market, Webkit has a much larger market share, and it absolutely dominates. Every single recently launched modern OS, with the exception of WP8 (which has a tiny market share to begin with), is using some Webkit-based browser. While it’s open source, this could lead to stagnation of the advancements Webkit will be making in the future.
Chrome started out by forking Webkit initially because they had a completely different multi-process architecture, that Webkit didn’t have at the time. In Webkit 2.0, they added some kind of multi-process technology, too, but it seems it’s different from the one Chrome is using, so Chrome had to support both.
Google believes that by forking Webkit, they can use up to 4.5 million fewer lines of code in Chrome, because they won’t have to support everything else that Webkit involves, that they might not need. With Blink, Chrome will be using only the part of Webkit that it needs. This will lead to a faster Chrome, and probably a faster Safari browser, too, in the future, if they remove the Chrome-related technologies.
The new Blink rendering engine will be available immediately in the development version of Chrome, and should be in Chrome stable starting with version 28, which should be around 3 months from now.