plus the paid for version has some extra features.

Simple User Interface
The Simple User Interface is used to start on-demand scanning, work with the results and change various options. Basic resident protection settings can be modified here.

The Simple User Interface is the main application of avast! 4 Home Edition. You can start additional avast! modules from here, such as the Virus Chest, Updater or Log Viewer.

The appearance of the Simple User Interface is very flexible. It supports so-called skins (changeable appearance of the application), so you can change the interface to suit you. The main package contains three skins, with additional skins available from our web pages.

Enhanced User Interface
In addition to the Simple User Interface, the Professional Edition presents the Enhanced User Interface, giving you access to all features and possibilities for extensive scanning control.

Unlike the Simple User Interface, the scanning is performed by so-called "tasks". First, you define a task, including various parameters - areas to scan, what to scan, how to scan, etc. Having the task, you can run it (possibly repeatedly). Each task generates a list of results, which you can work with later.

Another key feature, closely connected with tasks, is the Scheduler. This feature gives you the ability to schedule the tasks running, either one-time or periodically.

Additionally, it is possible to set all existing scanning options, a feature not available in the Simple User Interface.

http://www.avast.com/eng/avast-4-professional-antivirus-antispyware.html#3