Well a straight answer to this question is “GNU/Emacs is a text editor” which can be easily extended and customized. But I would say its an Operating system in which one can do anything from managing other processes, files, folders, check mails, browse web and even listen to music apart from just editing text and writing codes. For developers and coders it acts as an full fledged IDE.
Newbies often get confused with the concept of GNU and Linux. Most people end up saying GNU/Linux as ‘Linux’. The important thing here to understand is most people don’t directly use Linux. This may sound contradict but Linux is just a core part of an operating system. It’s task is to handle underlying hardware.
Users work on application, and applications can’t talk directly to hardware, they take help of Linux-kernel. Major Free and Open source applications(Libre Office, VLC media player etc.) are licensed under GNU License. In fact the basic core-utils packages used in Linux like copy, move etc. are GNU applications.