Reporting something
What happens after you press it, and what it does not do.
Report is in the ⋯ menu on any post and on any profile. Reporting a profile is for something about the person; reporting a post is for that post.
What happens Copy a link to this section
It goes to a person who reads it. You are told when it has been looked at.
What it does not do Copy a link to this section
Reporting does not hide anything by itself, and it does not block anybody. Those are separate on purpose — you may want one, the other, both or neither, and bundling them means somebody who wanted to report quietly ends up making a statement they did not intend.
You can block from the same menu.
Reporting a Today post Copy a link to this section
Worth knowing, because Today posts are deleted after a day: if you report one, a copy nobody can see is kept until the report has been dealt with. A report about a post that no longer exists cannot be answered.
Writing a useful report Copy a link to this section
Say what is wrong with it. A report that says only "this is bad" needs somebody to guess what rule you have in mind, and they will often guess a different one.