Tar, like IAP stated, contains carcinogens. It has been strongly suggested that THC cancels out carcinogenic effects by killing precancerous cells.

The main thing people forget is that a heavy smoker of resin has a much higher risk of developing chronic bronchitis, decreased lung capacity, and the worst of them all, emphysema. Emphysema is when your alveoli, the small popcorn things in your lung responsible for gas exchange lose their elasticity because the inhalation of smoke with more particles (resin) damages the elastin and collagen of the alveoli. This leads to a constant and irreversible feeling of not getting enough oxygen; patients I've seen with advanced emphysema can't even describe how terrible their quality of life is. Of course, I am kind of going down a slippery slope, but it can happen.

So please, try to limit smoking resin. It may get you stoned, but it's certainly not worth complications which can develop later on in life. Save up your kief instead!