Does Nethunter overwrite the Android operating system? Or is it just an application?

I'll be buying a new phone soon. And I'll probably go with the Nexus 6. I was curious if Nethunter is a full operating system and thus replaces the android operating system or if it's more like an "app". I would be using this phone for daily use, and therefore it would be a waste of money if I can't just use the phone as a normal phone.
I have read that it's an application on your android device somewhere on an article but I want to be sure before investing in a not so cheap phone.

Thanks in advance

Nethunter not replaces the android OS. you can use your phone as normal way.

and can you tell me , is there possible to install on for example Uniscope XC2S there is installed Android 4.2.2 KitKat ,will it corrupt work of that phone??? there is 1.7GHz MTK6592 Octa-core CPU and 2GB RAM 16GB ROM ,will it be enouth? and can nethunter be installed on TF card ?

@Mattice: NetHunter is completely in different lane. Thats not a normal OS for the Nexus or one plus. Its not an application. You need to install Nethunter and Android 5 or 6 or etc both with dual boot spec. @karencho, you both need to have deep technical understanding to figure out what you aimed for ;)

In my experience with Nethunter 3.0 on Nexus 9 Marshmallow, it does not replace the Android OS, it just enhances it. So you could still use Android the same way as before, without even noticing that Nethunter is there (except for the boot animation and wallpaper).

One correction is needed: since official OTA updates rely upon your system partition being in pristine state, they will fail after you install Nethunter. That's why using AOSP is recommended.