Installing applications can be easily performed via Ubuntu Software Center, utility that presents the user detailed app-related panels (containing informations, of-origin website, images, video clips, etc), where installing an app is 1-click away.
Nevertheless, while the graphical above-mentioned method is clear, reliable and effective, there are cases demanding apps installed via the terminal (with commands like sudo apt-get install xapp) and/or updating, upgrading, etc, approach fully met by Ubuntu's default apt-get.