Why two autofocus switches?

Firs, t be nice, i'm new at this, and although i'm learning i just need some tips

my Nikon D7100 has two autofocus "switches" one on the camera and one on the lens

why two? How do they differ?

That is because this camera is capable of working with older style AF-D lenses which mechanically link to the autofocus drive in the camera body. Those lenses have no switch on the lens to select or deselect autofocus--the decision between manual or auto focus gets made by the camera body switch.
The AF-S lenses that have their autofocus drive in the lens have their own switch on the lens. Assuming you have all or mostly AF-S lenses, leave the body switch on autofocus, and use the lens switch to turn AF on or off. If the body switch is on manual, the lens will behave like it is on manual regardless of how the lens switch is set.

Cameras like the D5300 do not have the body AF switch, because there's no body AF motor, and AF-D lenses will always be manual on that body.

I also am not a Nikon user, but I'm quite sure I can answer this. Any Nikon pros feel free to correct me if wrong.

Your camera has a built in focus motor, the lower end cameras do not. There are lenses made for each type of camera. The lens you have on your camera is for use with the bodies that do not have the focus motor built into the camera. The focus motor is built into the lens itself, thus the switch on the lens to turn auto focus on and off. Lenses designed to be used only on bodies with the built in focus motor would utilize the switch on the body to turn auto focus on and off.

steve

ADD: I see someone else answered before me, but it does show I was correct.

I don't own or use a Nikon D7100, so give me a few minutes to download the owner's manual (see nikonusa.com) and find out.
As far as I can tell, the one on the lens should be set to match the capabilities of the camera to which it is attached.