Curation is everything an individual user or a Project on the blockchain does to engage with content.
That is, seeking content, resteeming, upvoting, and downvoting.
Curation is when you gather information about the subject, appraising value to the subject and weighing your decision with set curation parameters. To dumb it down, if you think this post is valuable for a specific niche, platform, or deserves attention, you promote it. The opposite can be done with flagging. Flagging is also a curation tool. When you think the post is not rewarded approximately to the value it contributes, you either upvote it or downvote it.
You have to make a difference between curator and moderator.
A curator curates, upvoted quality posts respecting certain standards given by the team he/she represents or his/her own, in case we're talking about independents.
A moderator can curate but can also remove posts if the platform has these tools. Chatting rooms have moderators who make sure users respect the rules. They can remove posts, ban and flag users etc.