Since 1999, I've been managing and running forums from the old Ezboard hosted forum days through to vBulletin and some of the key rules on my forums has always to keep discussions civilised (basically you can agree to disagree if a person(s) opinions differs from your own) and with the absence of personal attacks.
Not everyone understands the difference between a personal attack and providing a negative comment or opinion in a civilised manner so I thought I'd provide an example of the difference below.
The example (and here) is in the context of this thread started at here (always good to have context ).
Basically the difference between example 1 vs 2, is that the first example, I speak and direct my opinions and commentary to each member's actions, behaviour or opinions. Whereas in second example, I direct it to the person(s) themselves and/or name calling which is either irrelevant to the argument or position I am conveying and/or has no evidence to back it up.
If the above example still doesn't help clarify what constitutes a personal attack, Wikipedia's own rules and guidelines have a more extensive list of examples posted here http://en.wikipedia.org/wiki/Wikipedia:No_personal_attacks. The most relevant part being the below: