Summarize this article in one sentence.
If you've received a copyright strike on your video, it's because the original creator or owner has decided that your video does not fall under "fair use." Fair use allows you to use content that others have created, but only in specific circumstances that are decided on a case-by-case basis. Fair use is a fairly complicated topic, but generally your video will be measured against the following four factors (in the US):  The purpose of using the copyrighted content. The video needs to add new expression or meaning to the original copyrighted content. Non-profit and educational uses are given more leeway, but are not exempt. If your video is monetized, your chances of claiming fair use diminish. The nature of the copyrighted content. Using factual copyrighted content (e.g. news reports) is usually considered more fair than fictional content (e.g. movies). The ratio of copyrighted content to your own content. You'll have a better chance of claiming fair use if you only use a few bits and pieces of the copyrighted content, and the vast majority of the video is your own work. Harm done to the potential profits of the copyright holder. If your video can be perceived to cause a negative impact on the owner's profits, you are less likely to be eligible for fair use. Parody is the main exception to this. A copyright strike will stay in effect for six months on your account. During this time, you'll lose access to certain YouTube features, such as uploading videos longer than 15 minutes. This is your only course of action if the copyright claim was legitimate and you did in fact violate a copyright when posting the video.  During the wait, you'll need to complete the YouTube Copyright School by watching a video and answering a few questions at youtube.com/copyright_school. If you receive another copyright strike during this waiting period, your six month waiting period will restart. If you receive three strikes, your account will be terminated. If possible, contacting the claimant may be quicker than appealing your strike. If the claimant has a YouTube account, use the private message function to send them a message. If a corporation or other entity filed the claim, you'll need to find and contact their copyright department.  Be polite when requesting a retraction, and clearly explain why you believe the strike was a mistake. Don't just say "fair use;" give evidence for why you believe the claim to be wrong. The claimant is under no obligation to retract the copyright strike claim. If you believe your video does not violate fair use, or you think the copyright strike was an error and you don't actually use copyrighted material, you can file a counter-notification.  This is a legal claim. By filing a counter-notification, the claimant will be able to see your personal information, and you open yourself up to lawsuits. The counter-notification takes ten days to process. The claimant may file a court order during this time to keep your video offline. If you decide you want to submit a counter-notification, you can do so from the Copyright Notices section of your account (youtube.com/my_videos_copyright). All of the videos that you have received strikes for will be listed here. If you see a "Matched third party content" or "Video blocked" message next to a video, this is a Content ID claim, and is handled differently than a copyright strike. See the next section for details. This will begin the submission process. You will be warned that you should not proceed unless you are prepared to take your case to court. You should only proceed if you are sure that your video should not have received the strike. Check the "I have read the above statement" box to reveal the form. You'll need to enter your real name, address, and phone number. This information will be made available to the claimant. If you have a lawyer, you can enter your lawyer's contact information instead. Enter in your reasoning for why your video falls under fair use, or why it was mistakenly identified. You don't have a lot of room in this field, so be clear and to the point. This will not be sent to the claimant. You can include a message to the claimant as well. You may want to reiterate why you're filing the claim, so that they can retract the claim if necessary. Avoid attacks in this message. This will make the form legally binding. You'll need to agree to all of the statements to proceed. The process takes about ten days to complete. If your video is found to be covered under fair use or was mistakenly identified, it will be restored and the strike removed from your account. If the claim is denied, the video will stay offline and the strike will remain. In very rare cases, you may be sued by the claimant to keep the video offline. If you are unable to submit an online counter-notification because your account has been disabled, you can send one via email, post, or fax. Include your name, address, phone number, video URL, and signature. Make sure to also include the two required statements on this page, where you can also find the mailing address and fax number.

Summary:
Ensure your video falls under "Fair Use." Consider waiting out the strike. Contact the copyright owner and ask for a retraction. Submit a counter-notification if you believe your video was misidentified or qualifies for fair use. Open the Copyright Notices section of your YouTube account. Click the "Submit counter-notification" link next to the video that was taken down. Confirm that you want to submit the counter-notification. Enter your personal information. Give the reason you're filing the counter-notification. Send a message to the claimant (optional). Check the boxes signifying that you agree, then sign your name. Click submit and wait for a decision.