Copyright is the exclusive legal right to reproduce, publish, sell, or distribute the matter and form of something (such as literary, musical, or artistic work). This includes photographs.
Copyright law traces its origins back to the United State Constitution. Article 1, Section 8, Clause 8 states "To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries."
The basic framework of the United State copyright law is established in the Copyright Act of 1976 and further detailed in Title 17. This gives photographers the exclusive right to reproduce their photographs (including making copies) and bans sharing on social media without the creator's permission. The Copyright Act also goes on to give photographers copyright protection from the moment the image is created. And, last but not least, the Copyright Act establishes copyright as a property right.
Just because you have hired a photographer to take images of your family, an event, a location, etc., does not mean you own the images. You are paying for the photographer's professional use of their time. Even after you purchase an image or package from the photographer, you have only purchased that set of images. Not the ability to reprint, make copies, distribute (via invitations, Christmas cards, etc.), or post on social media.
In order to legally be allowed to reprint images, share images publically, or pass out copies of the images you must have a signed print release from the photographer stating it is ok to use the images. If the image was taken for a school, sports team, or other types of groups, contact them and see how you can use the images.
Reproducing photos without permission can result in civil and criminal penalties.