Trolling

Griefing is the act of destroying other people's creations on a multiplayer server. It is frowned upon by the majority of Minecraft users. It wrecks construction, ruins many hours of work, and makes levels untidy. At best, griefing annoys other players, at worst it can destroy an entire server.

Possible Survival Griefing
See here.

Random Destruction
This is when a griefer destroys random blocks, putting holes in walls and other structures, making them look broken and messy. The way to combat this is to just have an admin kick the offender, then repair the damage by hand. If there is no admin to kick, then just wait for the griefer to get bored (though on some servers are possibilities to vote a player to get kicked).

Random Construction
This is when a griefer creates random blocks on constructions, making them look messy. The way to combat this is to just remove the extra blocks and have an admin kick the offender. If there is no admin to kick, again, just wait for the griefer to get bored. This type of griefing doesn't seem as bad as the first, but it can be just as annoying. Often griefers both destroy and create blocks in their quest to annoy others.

Water Flood
This occurs when a griefer destroys blocks causing water to spill across the level and into/over any constructions players have made. This normally can only happen when the construction is underground, or the ground level is below the level of the ocean. There have also been instances of hackers making water blocks in the middle of the sky, causing it to flood the entire level. If the damage is minimal, or the area small, a player can fill the area with another type of block, such as leaves, then dig them out again, leaving the area dry, or use the sponge block. Any water nearby will re-flood the empty area again however. Another danger that water also causes is the huge number of chunk updates that occur when water spreads rapidly can cause buffers to fill and players to lose connection to the server.

Lava Flood
Similar to a Water Flood, a lava floods the area with lava instead of water. Lava is far slower than water, so most of the time, if it's caught the lava can be stopped with a few well placed blocks. Lava floods are also rarer because lava is mainly found underground, at depths far below the common areas of water. However, well-planned lava floods can be catastrophic, because sponge blocks do not absorb lava, so the area must be cleaned out by hand by displacing the lava with blocks and erasing them, drying off the area.

Offensive Constructions
Some griefers often grief others by writing rude words or drawing phallic symbols everywhere. It's often generally harmless fun, but if a player finds it offensive they can destroy the offending problem, hence the griefer has become the griefed. This is far less of a problem than the others, although some constructions high up in the sky may take some creative thinking to remove.

Flattening
A relatively new and rare form of griefing but still worth mentioning. Flattening is exactly what it sounds like, destroying (or in some cases adding) blocks until the ground in an area is completely flat. While random destruction can cause a level to look ugly, flattening completely removes the structure from existence as if it were never there. This form of griefing is much harder to perform and is usually done while servers are empty or have no admins present. The best ways to defend against this kind of griefing is the use of spawn prisons (although these are generally ineffectual) or shutting the server down when it can not be watched.

Tools of The Griefer
This is a catalog of the different methods or programs used by griefers to perform the aforementioned tasks.

Hacks
Programs that manipulate Minecraft in a way that was not originally intended by Notch.
 * Flying - Now in widespread use by both griefers and admins, this is a common hack and can allow griefers to easily reach tall structures and begin deconstruction of areas that will be difficult to reach by players lacking this hack.
 * Auto-click - The staple tool of any griefer is the auto-click which allows the player to quickly and messily place or delete blocks. When combined with increased speed this can be a destructive combination.
 * Noclip - This means to turn off all collision with other blocks, usually used to bypass unbreakable stone. This can be done in several ways. This ability is very common.

Social Engineering
Not used by most griefers, but if done correctly it can be extremely confusing to figure out who the griefer is among a server full of people. Social Engineering, in the context of griefing, is convincing admins or other players that you have good intentions and are to be trusted. While most griefers use more of a guerrilla strategy, this sort of griefer is significantly more dangerous because of the confusion they can cause.

Time
Some griefers will wait in a server or return to it when there are no admins online, at which point they could destroy until another admin logs in. It can be very stressful to regular players when this occurs as all they can do is watch as their creation is destroyed before their eyes and may cause players to lose faith in a server they frequent.

Bots
The most destructive tool a griefer can ever employ. Very uncommon and they always come in swarms. Not much is known about exactly how they work or their full capabilities but these bots are very dangerous and are generally only employed if a specific server has been "targeted". The bots can make very human like responses and it is suspected that if the bot swarm is being overseen by a hacker, the hacker can speak for the bots when they are questioned. These bots come complete with many macros which can instantaneously create pyramids or massive holes in any structure; they also fly around and employ random destruction griefing. IP banning and kicking are useless as the bots switch IPs and account names in seconds and come right back, the only real defense is to shut down the server and wait. Some of the behaviors bots have been seen to exhibit are:
 * Not speaking unless spoken too (will not ask to be let out of spawn prison)
 * Using long, misleading names that are difficult to type (use the tab menu and click their name to make it automatically appear in your chat box)
 * Frequent jumping (although this is exhibited by many human players as well)
 * No clear purpose or destination, they wander (Many human players do this as well, to explore the server's existing creations)

Combating Griefers
The best way to combat griefers is to kick and ban them, often with a permanent ip ban. Until there is a more robust system, however, players and admins just need to be vigilant. When a griefer is found, an admin can verify what they are doing by teleporting to them. After that a /kick or /ban command will remove the griefer from the server.

If an admin is not on the server, or is handling a heavy load of griefers, a good way to combat the griefer is to rebuild things as the griefer destroys them. This serves the dual purpose of repairing damage as well as causing anger to the griefer by effectively undoing what they have undone. A taste of their own medicine so to speak, although rebuilding something properly takes more time than destroying it. It will not be effective to ignore the griefer. Simply ignoring a griefer will cause them to partake in more large scale destruction such as flattening.

Griefer swarms
While one griefer can be bad, on rare occasions they may attack in groups. Sometimes members get to the point of blaming others in the group to avoid being caught. They can also attack rapidly, typically abusing vote bans to clear out the server. A typical sign is about four people(four is a common vote ban threshold) people joining at once, but that can be coincidence.

The swarm also provides the problem of taking longer to ban.