Griefing

Examples of griefing include: the destruction of structures built, owned, or both by other players, stealing other players' items, and repeated killing of less well-equipped players. Many servers use plugins such as PlotMe or WorldGuard to allow players to protect their creations. Such systems are rarely fool-proof, but provide protection against all but the most determined attacker. Even if a server does have a plugin to protect land, some people do not claim land and get griefed.

Griefing is not always banned on servers. On some, mainly faction servers, it's the 'status quo', and if this sort of gameplay is not to one's liking, these servers are not a good choice. A good example of this is 0b0t.org

If one hasn't already, check whether the server one is playing on has any or updated protection. A good thing to check for is whether the server has rules against such behavior, or you might get punished. In such cases, if one does get griefed, one may be able to claim some sort of compensation for one's loss.