Pretty simple and lots of fun. Most paintball guns shoot a .68 caliber paint pellet. The pellet is a thin gelatin shell filled with a water soluble paint. Guns shoot the pellets at a speed of 200 to 300 feet per second. Safety is highly stressed at most fields.
Many fields require goggles be worn at all times unless in designated areas.
Ignoring safety is a good way to get yourself ejected from a field. The paint pellets break open upon impact, and generally cause very little pain. What pain is experienced is more of a stinging sensation from the pellet's impact against the skin or through clothing. In my experience, the stinging pain goes away after a few seconds. Of course, the closer you are to the muzzle of the gun, the higher the velocity of the pellet when it hits you.