What is Copyright
A Copyright as defined by US law is:
Original works of authorship includes a wide range of content including books, games, movies, videos, characters, lines, and even sound effects. Copyrighted material that has been registered with the US government can be searched here. Copyrights generally last until 70 years after the author’s death, or if the work was created as a work for hire, for 95 years after the first publication or 120 years after creation, whichever comes first. Once a work’s copyright expires it enters the public domain and loses all copyright protections, this is why you can use classical music like Mozart without any issue. Copyrighted material is a type of intellectual property, a broad term referring to protected intangible assets. A more comprehensive definition can be found in the video below but it's basically something a person or company owns that is not physically tangible and is meant to cover things not covered by conventional property rights.
Copyrighted holders maintain several important rights in regards to their material. They maintain the right to: reproduce, prepare derivative works, distribute, publicly perform, publicly display, and perform publicly in a digital environment (read the relevant portion of the statute here). This means that taking one of these actions in regards to the copyrighted material of another person without permission will violate copyright laws. So, using a copyrighted movie like Disney’s Encanto as an example, Disney maintains the right to reproduce and distribute the movie so you can’t record the movie (reproduce) and post it to YouTube (distribute). You can’t make a remix of the songs in the movie (prepare derivative works) or play them on a jumbotron at a football game (publicly perform). Finally, you can’t stream the movie to a group online (perform publicly in a digital environment).
There are two main ways to legally use copyrighted material: obtaining permission from the copyright holder (typically by purchasing a license), or engaging in ‘fair use’ of the copyrighted material
A license is a right to use something owned by another person or organization. Licenses widely vary from case to case and are largely dependent on the specific terms written. For example, you can purchase a license that gives you the right to install and use Photoshop on your personal computer. This license has various terms, like requiring the use to be personal not commercial, the software can only be installed on one computer, the license lasts for a year and needs to be repurchased afterwards. All of those are common clauses in license agreements, but there are many more and all license agreements should be read thoroughly to understand them.
What is Fair Use?
Under US law:
These circumstances are more clearly defined in section 107 of the copyright act, but generally depend on if the work in question is:
Not for profit or for educational purposes
The nature of the work, whether creative or more factual
How much copyrighted material is present in the work
The effect the work will have on the market for the copyrighted work
Fair use is why I can directly quote this Stanford article describing fair use:
Even though I am reproducing part of a copyrighted article of another and distributing it on our website, it is not copyright infringement or illegal because what I am doing is fair use. This website is not making money, is for education purposes, and I am using only one sentence of the article, all of which support a finding of fair use.
Fair use means that things like movie reviews are generally fair use due to their creative nature, but can violate copyright if there is too much footage of the actual movie within the review. It is also why remixes of popular songs are not considered fair use, as they are derivative works and impact the potential market for the original song.
Unfortunately for creators who use copyrighted material, fair use is generally evaluated on a case-by-case basis so there can be inconsistencies in how it is enforced and there is no way to know for certain beforehand if something is fair use.
Further reading:
https://www.copyright.gov/fair-use/more-info.html
https://fairuse.stanford.edu/overview/fair-use/what-is-fair-use/