Everything you post on the Internet (on Facebook or commenting on a video on YouTube) stays there forever - even if you delete it. When you upload a file, it is put onto a server. Then you access it again later, you actually access the file that is on the server. That means, that you don't have DIRECT access to that file anymore, you can only view it. That means that if you want to delete it, you may not be able to completely delete it.
So, reconsider posting something mean or rude online. It may be seen by someone you don't want to see it - like a potential employer when you go to work someday. The Internet is ever lasting so it may be there for a very long time.
Be an ethical person online. If you wouldn't say something to a person's face in real life, then you shouldn't say it online either. Many people feel like they can say whatever they want online, since the intense face-to-face confrontation is absent when messaging others online. However, bad-mouthing and sending another person rude messages online is cowardly. You should rather resolve conflicts face-to-face. If another person or "bully" is threatening you online, tell someone you trust to help you resolve the issue (see Trust someone close to you).
So, whenever you write something to another person online, ask yourself whether you would say this to their face. Reread your message and ask yourself if this message could possibly depress the receiver. Also ask yourself whether you would have appreciated if the other person had sent this message to you. If you answer "no" to any one of these questions, you should rather not send the message.
Cyber bullying is the act of sending mean and rude messages to another person (or people) with the intent of making them feel bad. It is also a leading cause of suicides among teenagers. That means some teenagers have even committed suicide over these rude messages. Would you really want to be responsible for something like that?
Think before typing something that can be potentially upsetting!