Griefing is best summarised as "the behaviour of someone who was told, from the day they were born, they're only here because the condom broke."
Despite what Trolls claim, their behaviour is no different.
Now let us clarify something here; when behaviour becomes griefing.
1) If you have to use the phrase "it's not against the rules" to justify it, you're probably Griefing.
Yes, there is no rule to say you shouldn't spawn-camp, but it makes you a **** to do it. Exclusively using the most overpowered and unbalanced weapons, perks, etc. does not make you an awesome player, it makes you a cheap one.
2) If the behaviour is detrimental to the team, it is griefing.
Anyone who needs this explaining should, in theory, be too retarded to actually login, so we can move on.
3) If it's bullying, it's Griefing.
If you are deliberately picking on a specific player, stop it, and stop being a twat.
A point worth noting here is there is a difference between picking on, and simply engaging. If I'm playing a Point Capture mode and the same guy keeps rushing me from the same direction, it is hardly bullying. If I am going out of my way to hunt down and kill that specific player, it is.
4) If you set out to 'get a reaction', it's griefing.
Seriously, if your behaviour is motivated purely by the desire to see/hear someone get really pissed off, you are a complete and utter douchebag. Do us all a favour and torch your gaming hardware and rid us of your pestilence.
Another thing I'd like to point out is the key difference between Griefing / Trolling and Pranking. A Prank is essentially a physical joke. A JOKE: something the victim will find funny. Case in point, I once went into a friend's tree farm and 'inverted' all the trees so it looked like they were growing down from the ceiling. Pointless? Absolutely. Harmfull? Not really - it just made it a bit trickier to cut them down. Funny? 100%.
Finally, do not confuse Griefing with Shits & Giggles. It is not Griefing to go onto a Hardcore COD match and insist on melee-killing the opposing team. Nor is it Griefing to play a game without memorising the map, asking Google what the optimum build is, and generally refusing to play 'hardcore'. Something that a lot of 13 year old COD players forget is that people have these things called 'Lives', and they demand a lot of our time and attention. This means when I do play a game, I want to have fun, and if that means bringing my favourite shotgun when you think I should be using the Loltard Rifle then tough shit.