The Rules of RuneScape (formerly called the Rules of Conduct, commonly just called the Rules) are part of the terms and conditions of playing RuneScape. When players make an account, they agree to be bound by the rules and to face punishment (i.e. a ban or a mute) if they do not abide by them.
Jagex's Customer Support team may apply a variety of sanctions against rule breakers, up to a permanent account ban. Lesser offences may result in a warning, temporary ban, or a temporary or permanent mute.
The RuneScape Forums operate under a set of rules called the "Code of Conduct". The Code of Conduct is very similar to the Rules of Runescape, with some sections tailored and expanded upon to make them more suitable for the Forums. You can find more information about the Code of Conduct in RuneScape Forums.