Climbers use chalk to absorb the sweat on their hands and fingers so they can grip holds more easily. Climbing chalk is basically Magnesium Carbonate, the same stuff you use to write on blackboards, supplied in climber friendly forms. It is usually carried in a small chalkbag attached to a belt or the back of a climbing harness enabling the climber to dip their fingers whenever they feel the need.
You can buy chalk in loose powder form, in a disposable ball, which is a porous mesh ball filled with chalk which comes out of the ball when you squeeze it, in a refillable ball (same as a disposable ball but you can refill it up when it runs out), in a solid block (which you crumble into your chalk bag) or even in a liquid (squeeze onto you hands and let it dry out).
Any type of chalk does the trick but most climbers have their own preference and stick by it.
There are a couple of things to consider when buying chalk. Some chalk contains a drying agent, which is very effective at keeping your hands dry, but can cause your hands to dry out and your skin to crack, especially if you have sensitive skin. Generally use of a good hand moisturizer after climbing sorts out this problem. The other consideration is that some climbing walls do not allow the use of loose chalk and insist on chalk balls as they can't spill in the way that loose chalk can.