A launcher is where you find your home screens, widgets, apps, etc. If you download and install a third party launcher such as Apex or Nova your stock launcher is still there. It doesn't change unless you change it. You can do whatever you want with the third party launcher and it won't mess with your stock launcher setup. They are completely separate. It won't use more battery. It doesn't really run in the background. Sheen you first install the new launcher and press the home button a prompt for which launcher to use will come up. You can select the such launcher or the new one. You can also chose to use the option you pick just once, in case you want to test it out and go back and forth between launchers, or always if you decide you like the new launcher more.
I personally use Apex, but many people like Nova launcher. Those are the two titans. But there's a dozens of different launchers that offer different features and UI experiences.
