A more thorough explanation:

Fair use is a defense in copyright law that allows someone to use a copyrighted work without permission from the owner. The law lists four factors that courts use to decide if a use is fair:

1. The character and purpose of the use
2. The nature of the original work
3. The amount taken from the original work
4. The market effect to the original work

Courts weigh these factors to determine if the use is fair. If it is, the person using the work is not liable for copyright infringement.

The first factor looks at whether the use is for commercial or nonprofit purposes. Nonprofit uses are more likely to be fair. The court also considers whether the new work adds something new or is transformative. If it is, it is more likely to be fair. For example, a parody or satire is a type of transformative work.

The second factor looks at the nature of the original work. If it is factual instead of fictional, it is more likely to be fair. If it is unpublished, it also favors fair use.

The third factor looks at the amount taken from the original work. The court considers both the quantity and quality of what was taken. If a substantial amount was taken, it is less likely to be fair.

The fourth factor looks at the market effect of the use on the original work. If the use takes away from the market or potential market of the original work, it is less likely to be fair.

Examples of fair use include copying an article for personal use, using a small portion of a song in a review, and using a copyrighted work in a parody. In each of these cases, the use is transformative and does not take away from the market of the original work.

Overall, fair use is a way for people to use copyrighted works without permission in certain circumstances. The court considers several factors to determine if the use is fair.