Is burning a flag a form of expression?

If someone burns a flag in protest, is it more a form of blatant shock and aggression? Or is it a form of expression, letting people know they disagree with the country's present status? And thus be "protected" as a form of expression? I can see both sides and am interested in gaining some other people's opinions. Please be open minded about the question, I'm not trying to offend anyone :)