YouTube

The "Spam" link allows the community to control the number of spam comments left on videos they upload or watch. If enough users mark a comment as spam by clicking the "Spam" link next to the comment in question, it becomes hidden under a "Marked as Spam" link. By clicking the "Show" link, you can see the comment again. Spam is content and/or correspondences that cause a negative user experience by making it difficult to find more relevant and substantive material. It can sometimes be used to indiscriminately send unsolicited bulk messages to users. Please use the "Mark as Spam" feature with extreme caution, as those who misuse it may be prohibited from using the site. Uploaders have additional control over comments made on their videos. They can choose to delete the comment after it's been made or decide to approve/disapprove a comment on their video before it's displayed. The video's uploader also has the ability to "unmark" a comment as spam. If you believe a comment you made wasn't spam but has been marked as such, you may want to contact the uploader about the issue.