In short
Report flag alerts moderators — use it for content that should be removed.
There are people who upload or post things that do not belong on a PG community site. Above posts (and in similar places around member content) you will find a report control — historically a small red flag icon.
Use it for content that should be removed (abuse, spam, illegal, etc.). Reports go to moderators; they are not a public downvote.
Related
I'm disgusted with what someone has written, thumbs up/down.