No griefing, including but not limited to:

Destroying blocks placed by other players.
Placing blocks on other player’s structures.
Flooding structures.
Spamming items.
Destroying crops without replanting.

Structures

Don't build very close to other players' structures; be sure to leave some space.

If you have a large project such as a city or megabuild, expand in directions that do not come into conflict with other players. Exhaust all available unopposed routes before expanding near already established players. If you must expand in the vicinity of other players, speak with them first and see if they are willing to participate in your project. Leave signs, /mail send, or /msg them in chat. If you are unable to come to an agreement, contact a moderator for assistance.


Land Claims

In order to claim land you must build a border around what land you want to claim. Borders must be on the surface of the land in order to be considered valid (no underground borders).
A claim extends from bedrock to skylimit. However, mine-shafts that intersect with this claim are not considered griefing.
If your claim goes without use for a long enough period of time an admin can choose to give it to a user who requests it.