Right of resale. The holder of a copyright has control over the initial sale of their work. However once sold, the new owner can sell the copy without getting permission from the copyright holder.
Library and archival exceptions: Libraries may loan copies of books and other materials which they have purchased without getting permission, including interlibrary loan. They may also make copies for preservation purposes Note that this has gotten sticky in the case of e-books, where the vendor may restrict loaning and copying by contract.
Persons with Disabilities: Copies may be made in new formats in order to enable access by persons with disabilities, e.g. creating audiobooks from printed materials.
Teaching: Copyrighted works may be used in face-to-face and some online teaching.
In addition to the exceptions above, there is also a broad category called fair use. Fair use exceptions are common, but there are no hard and fast rules as to what constitutes fair use. Instead, the courts have established guidelines.
You must get permission. This may involve paying the copyright holder a fee...or not.
Ask the copyright holder directly. It may require some research to track down the author. If the copyright is registered with the US Copyright Office, they will have the author’s contact information…at the time of registration, anyway.
For performative works (plays, choreography, music, etc.), there is usually some company or agency that handles the licensing. In the US, the American Society of Composers, Authors and Publishers (ASCAP) is the main licensing agency.
For scholarly journals, there will usually be a “permissions” page with the information you need.
Check with the Copyright Clearance Center.
Or you may be lucky and the copyright holder has automatically licensed the work, at least for certain purposes.
Copyright violation is a legal matter – use of copyrighted materials without permission of the copyright holder.
Plagiarism is an ethical matter – use of another’s works without proper attribution, in effect claiming them as your own. Note that plagiarism can be a serious violation of rules at an academic institution.
You can violate a copyright even while giving proper attribution.
You can plagiarize a work which is in the public domain.
Posting copyrighted material (text, photos, videos, audio material) without the permission of the copyright holder is a violation of copyright law.
Social media sites, in general, will take down material that is in violation of copyright, though in some cases they must be notified.
Note, however, that posting a link to a legitimate source of copyrighted material is completely permissible.
Posting your own material is permissible. You hold the copyright to any original works which you publish on social media
However, you are generally granting permission (a non-exclusive license) to the social media site to make use of your work. To be sure, read the copyright policies of the site you are using, and note that these may change at any time.