Xenial Xerus is an LTS version of Ubuntu. LTS stands for Long Term Support. LTS versions come out every two years (instead of every six months like regular versions) and are supported for five years. So, in the case of Xenial, that means support up until 2021. Ubuntu 14.04 was the LTS before this latest release, and, although it will be supported with updates and security patches until 2019, it is probably a bit long in the tooth by now. Even if you didn't update in 2015 at all, you may want to update now.

Another thing that was getting long in the tooth and has gone through a facelift is the command-line-based APT software manager. Although the old syntax is still there, instead of searching with

apt-cache search <somepackage>

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