I suppose all of you know about upvoting. Upvoting is what gives a post monetary value. Upvotes are to distinguish valuable content from... less valuable content.

Downvoting is equally important for the #steem platform. If ypu think of upvoting as the carrot, then downvoting is the whip. The purpose of downvoting is to discourage malevolent content, content that is bad for steem as a whole.

How to

Every post has a small flag at its upper right, sometimes with a number alongside it. Click on the flag to downvote the post. The number, if present, indicates how many downvotes the post already has.

What to

The most important kind of bad content is SPAM. It clutters up the list of posts, without adding value to the steem network. Please help downvote SPAM to keep our platform clean!

SPAM comes in different varieties, and sometimes it is even difficult to recognize.

• A post may appear to have interesting content, but is crossposted over many tags that have no relationship whatsoever with the post's content. That's tag-spamming. If you browse steem by tags you don't want to see that funny movie under "cooking", "investments" and "the-hack", for example.
• There are automated scripts in the wild that simply repost articles from news sites etc. into steem. That's not the point of steem. If people want to read a newsfeed they can go to the news site directly. Note that I'm talking about unsystematic copying here. Comments on articles, or a selection of articles may be OK - YMMV.
• Closely related to the latter is plagiarism. People want to post as much as possible to make money, and because their creativity has its limits, they copy content from elsewhere. The general opinion within the steem community appears to be that copied content is OK, as long as due credits are given plus a link to the original source. I think copying stuff is generally bad, but that's just my opinion.

There's other types of content to downvote, too, of course. Opinions may differ, depending on your cultural background. In general, try to be liberal, but also be considerate.

• Sexual content should be tagged "nsfw", because steem is easily accessible to minors, and may be read by people in their workplace. If you come across sexual content that is not tagged nsfw, please downvote it.
• Hate speech is a tricky subject. A line has to be drawn somewhere between the right to free speech, and the rights of others affected by such speech. Read the Universal Declaration of Human Rights when in doubt, and downvote accordingly.

What not to

Since downvoting is a kind of censorship, it must be applied with care.

• Do not downvote content that expresses a different opinion than your own. Its author has the same right to free speech that you have. Feel free to post your own opinion in a comment instead, or in a separate post.
• Do not downvote content simply because you dislike its author. In particular, don't downvote a post just because its author downvoted one of yours. That's childish.
• Remember that many (most?) steem users are not (english) native speakers. Don't downvote posts for bad spelling and/or grammar. We're not at school here.
• Some content may be illegal in your jurisdiction. Remember that steem is a global network, and that the content in question may be legal elsewhere. IANAL, but downvoting such content may get you in trouble, because the downvote is proof that you have seen (and thereby downloaded) that content.