Liferay Portal was specifically designed to be extended and modified, beyond what even most open source projects provide. Though its source code is available, Liferay Portal’s developers have created many extension points so you don’t have to customize the source to make Liferay Portal yours.
The first (and easiest) way of customizing parts of Liferay Portal is with Application Display templates. These let you change the way built-in applications look. For example, if you don’t like the Documents and Media Library’s file manager view with large icons, you can create an Application Display template that shows documents in some other view. If you don’t like the layout of the Blogs application, you can change it so that it has the look you want.
Liferay Portal goes far beyond this, though. Special software components called modules enable developers to change Liferay’s interface and behavior without having to modify any of Liferay Portal’s source code. This provides you all the benefits of building your site from scratch, but without all the effort to actually build from scratch. If you want to make a change to the user registration screens, add support for a proprietary single sign-on mechanism that you’ve written, add a feature to the message boards application or anything else, you can make those customizations. And if you’re a developer, you know that it’s a whole lot easier to customize something that almost does things the way you want than it is to write that feature from scratch.