k then.
If you are new to Minecraft, you must expect these people who will, and I repeat WILL annoy you like hell.

Griefers
Annoying people who come on servers and wreck havoc everywhere they go.
They destroy your whole entre server. If very unlucky, they might go and raid all of your stuff and then destory your server.

Griefers come when you're not online, or not guarding your area. Once they see that you're gone, they will go and destroy everything. They may even go in groups and do it.

Since 2012, people have added plugins to protect their servers from getting griefed. From having permissions to use easy griefing tools, to banning the use of them all together. Unable to break other users blocks to spying on them without their consent. Yes, griefers are that much of a pain that we had to make things to stop them.

Short post, gotta go somewhere.
- Choc