What is flagging? How can I flag a post/comment for the moderators to look at?
Members have the ability to flag posts and comments for a variety of reasons. Flags alert the mods to problem posts and comments, and indicate the level of severity of a problem post.
To flag something, click the [!] link by the post or comment.
When you flag something, the flag goes into a queue read by the mods. All flag reasons go into the same queue, so don't get stressed about the exact reason for flagging. If you think your reason for flagging will be hard to understand (for example a non-obvious problem with a link) you can use the contact form to explain.
Moderators will look at every flagged item, but not every flag will result in obvious action. Even those flags can be helpful, though, for example putting a contentious discussion on the mod's radar, or allowing the mod to email someone behind the scenes.
There is no need to flag 20 comments in a thread, because a moderator will read the thread when checking out the first few flags; please flag only the most important. Flags are generally used for current comments and threads, so flagging old posts and comments doesn't usually result in any action. Here is a good summary of when to consider using the flag feature.
FIAMO stands for "flag it and move on" and is a good first step with problematic posts or comments.