installing applications in Ubuntu

Ubuntu does not come with a preinstalled compiler such as gcc. If you ever want to gain that functionality you need to execute sudo apt-get install gcc in the terminal. After you do this, if you want to install an application from source usually you extract the file to a folder (tar xjf for .tar.bz2 and tar xzf for .tar.gz) and then run the following commands:
cd /path/to/folder
./configure
make
make install

(PaulStamatiou)

Ubuntu reposes

AddingRepositoriesHowto, LocalAptGetRepositories – Ubuntu Wiki (sometimes you need not only one package, but you have to provide a dependency like a library or whatever. When those dependencies are not in the official repositories, the only way to fulfill those dependencies is to put your package inside the repository.), AddingMultimediaRepositories – Ubuntu Wiki, How to Use Ubuntu Backports Correctly, BreakMyUbuntu (modeled onBreakMyGentoo) & PackageManagementHowto.
be careful, the repositories listed at apt-get.org are at the absolute bottom of the support ladder, and can include illegal or even malicious packages, and sometimes tend to fail even in their native Debian environment. [x]