The Ubuntu Software Center provides a mechanism to install many different applications and libraries to your Ubuntu desktop without requiring 'su' or 'sudo' (administrative) access. More information can be found at the features page here
Permissions and Troubleshooting
If your Ubuntu machine is UMIACS-supported and you are the primary user, access to the Software Center should have been granted to you at the time the machine was installed. If you find that you do not have adequate permissions to install software from this portal, please send mail to the UMIACS Helpdesk, email@example.com. Please provide both your hostname and username.
Accessing in Ubuntu 12.04 (Precise Pangolin)
In 12.04, one way of launching the Ubuntu Software Center is by using the Dashboard. The dashboard can be opened by typing the meta key (commonly referred to as the Windows key). This will provide you with a search box. Type in "Software Center" and it should be within the first few results. You can alternatively launch it from the terminal by typing "software-center" (if it's not in your path, the location is /usr/bin/software-center)