Chalk is that white powdery stuff you see in many gyms. You can put in on your hands and it will increase the adhesion of your grip to the bar. I like to use it with any exercise in which I feel my hands slipping on e.g. chin ups, wide grip bench press, cleans and deadlifts, etc. The more important the lift, the greater attention to detail I would take in applying it.
Competitive powerlifters will also use chalk on the upper back to reduce the chances of the bar slipping down off the back.
Powder
Powder (as in baby powder or talcum) can be used to reduce the friction of the bar on the body. Competitive powerlifters use it on their thighs so that the bar will slip up. Just be care ful not to get it on your hands or else the bar will slip out real quick!