To flame tag is to indirectly insult another person, unlike a straight personal attack. A flame tag indicates an offense without really pointing it out, I'd say that flame tagging can be far by worse than direct flaming.

"Why the heck would you start a topic asking about that?"

That could be a flame tag, leading to that you're stupid since you started a topic that I didn't like. Flame tagging can be hard to spot because it's all about how you read it. Most people mistake my text for flame tagging because they assume the worst of me.

A good example of a flame tag would be that, "I find that certain people have no humor at all." This is easily directed to the above who posted a poor fake edit. (No offense, of course. :P)