A griefer is someone who derives pleasure from ruining the good times of as many other people as possible by whatever means are available. On the low end of the griefer scale are people who inhibit movement and such, on the high end are technically sophisticated people like the ones who brought down the whole game the other day. The best way to avoid being a griefer is to apply the golden rule. Prior to taking any action ask yourself "would I want someone to do this to me?". If the answer is no and you do it anyway, you're on the road to griefer status. I think that the fact you even asked this question is a sign that you're pretty unlikely to become a griefer.