In Ubuntu, .deb is the default file extension which stands for debain. The .deb extension is like .exe extension in the Windows. I am going to demonstrate by installing Skype 2.2 Beta which is in .deb extension. I have downloaded the installation file which is of .deb extension.
Locate the file double click it, Ubuntu Software Center will open then click Install. You need to give authentication and the software starts to install.
Installing through Terminal:
You can install the file with .deb extension with the terminal in a very easy way. First, open the terminal by typing: Ctrl+Alt+T.
After that, go to the directory where .deb file is kept. I would recommend you to see 8 linux tutorials for beginners
sudo dpkg -i package_file.deb
Im going to install skype so I would type: