Good question scarletshadowz!
Basic thing is: If it looks like it was made by someone, don't mess with it unless you were given permission!
Stuff like breaking blocks off buildings, taking crops from organized looking farms, opening chests that aren't yours, etc.
Think of it this way: If it looks like someone put it there, chances are if you mess with it it's grieving.
Other types of grieving:
Begging
Spamming Chat
Foul language in Chat
creating ways for players to get hurt (traps)
creating ways for players to get stuck
creating ways to take players money or items
exploiting bugs
using client mods for your own unfair gain (xray texture pack is off limits, for one)
Use your best judgement, and if you think there's a possibility that something you think about might be grieving, ask! If a moderator is not online, wait to do it until one is online, or post in the forums to ask!
This is a great and fun place! and if we work together to keep it that way, banning won't have to happen ^_^
Oh another silly one:
Leaving half harvested trees in mid air!! Pet peeve of mine

haha