You will have the basic image that Dream supply.
Basically you have a Linux computer that has been set up to act as a satellite receiver.
The receiver is programable several groups of clever users have designed their own software that you can load adding different looks and functions.
They allow the receiver to do things not included in the original image. This could be software that acts like a CAM (Conditional Access Module) for decoding channels using a card (or without a card in some cases). To play games, add subtitles or EPGs. Different images do different things - best to check the most popular ones.
The images are available from several Dreambox sites and full instructions on how to download and install are in the guide at the top of the Dreambox section here.
Stickies
Please read that guide before you attempt to install anything on your Dreambox.
Have fun - all the help you need is here - just ask.