A "signature" can be any unique mark and, yes, it most certainly IS a form of copyright protection.
Deliberate removal of a copyright protection can result in fines of up to $10,000 per instance (in addition to other fees and penalties for misuse of the images).
Protections I use for my copyright include....
1. Keep any images posted publicly on the Internet small... normally no more than 800 pixels on the "long side". This is too small for printing or most other uses.
2. Watermark OR sign all images posted publicly online.
3. Record copyright ownership information in every image's EXIF.
Regarding #3, I have my cameras set up to record the basics and Lightroom set up to automatically add more info during Import.
You can probably set up your D7100 (not a model I use) to record your name and perhaps a little more basic copyright ownership information.
I have images "stolen" for use on Facebook, etc. all the time. Rather than fight this (which is fruitless), my watermark is essentially an advertisement with the URL for my sales galleries.
My watermark:
My signature:
Both the watermark and the signature are applied to images the same way. I now use the watermark feature while Exporting images from Lightroom. In the past, before LR was able to do that, I used FastStone Photo Resizer (a freeware) to add the signature or watermark. That worked well, but so does LR now and it streamlines my workflow.
My "Proofs" are always watermarked. Those are the "thumbnails" in my galleries (limited to 800 pixels max, long side). Most are event photography.
Images that are licensed to be used editorially or in advertising or in most other commercial manners are normally NOT watermarked or signed.
Images that are destined for personal use and display either online or as prints are always signed.
Digital files licensed for any use have the details of the licensing agreement embedded in the EXIF. If provided on a disk, I also will usually include a separate .txt file with the same licensing info.