Adobe Flash Player is the most widely used web browser plug-in. It enables a wide range of applications and content on the Internet, from games, to video, to enterprise apps.
The traditional browser plug-in model has enabled tremendous innovation on the web, but it also presents challenges for both plug-ins and browsers. The browser plug-in interface is loosely specified, limited in capability and varies across browsers and operating systems. This can lead to incompatibilities, reduction in performance and some security headaches.
More here on the Chrome Blog.