They cannot be easily included in Ubuntu by default due to potential copyright issues and so you'll have to install them yourself. Fortunately, it's not a difficult task. Just do the following:
$ sudo apt-get install msttcorefonts
...and everything's ready to go once this package is installed. Restart your Ubuntu machine once this package has been installed just in case.
There's more to fonts than that of course, but this is sufficient for most people. There's always the Ubuntu Guide if you feel the need for even more fonts on your system.