Since Ubuntu 10.04 (Lucid Lynx), Skype is part of the Canonical partner repository. To install Skype first you need to add the Canonical Partner Repository. You can do this by running the command

sudo add-apt-repository "deb http://archive.canonical.com/ $(lsb_release -sc) partner"

Then install Skype via the Software-Center or via the Terminal.

sudo apt-get update && sudo apt-get install skype

It is highly recommended to use the package provided in the Canonical partner repository, not the one distributed from the Skype website, as the Skype website currently points users to the wrong package for 64-bit systems of Ubuntu 11.10 and above.

Refer to this community documentation for more details in case of any problems.