The term "netiquette" is used to describe an informal set of ethical principles and cultural norms that substantially constrain the actions of most participants on the net.
You can't be put in jail for a violation of netiquette. But you can be shamed by other members of the online community.
Some violations of netiquette prompt other members of the net to take retaliatory action, such as sending large volumes of unwanted email. Such vigilante actions are themselves viewed as violations of netiquette under some circumstances, however.
As a practical matter, customary practices and group norms have very substantial impacts on the behavior of many people. Activities that are widely viewed as impolite or antisocial are less prevalent online than they might otherwise be because of the general desire of net participants to act as "good netizens".
Netiquette helps substantially to mitigate or reduce the following problems: