I see that tomlukeywood has answered the orignal questions, in addition:
It is not “app get”, it's “apt-get”, as you wrote correctly in the message body. APT stands for “Advanced Packaging Tool”. “app” is ostensibly a shorthand for “application”, but in practice it is a marketing trash word (“buzzword”) roughly synonym with “computer program”; used especially by proprietary software developers. I guess it's also another way of making fun of the ignorant clients who happily surrender their computing rights and hand their personal information to them (the developers).
We have been using “computer program” for decades and it works perfectly, it means what it says. On the other hand, “app” is sloppy language, because the speaker doesn't ever bothers to write the full word, which is usually incorrect anyway. In informatics, an application is a generic way in which technology may be put to use, for instance, the world wide web, e-mail, optical character recognition, peer to peer file sharing, and so on are applications. Implementations thereof are not applications but computer programs.
According to Wikipedia, “app” is a shorthand for “application software”, which is such as vague concept so as to be nearly useless (There is no clearly boundary between “operating system” and “application software” because any one would be arbitrary).
