Paintballs are the ammunition of a paintball gun.  The paint is non-toxic and bio-degradable.  When they hit, paintballs break open and ‘mark’ your opponents with paint (this is why they call the guns ‘paintball markers’).  They generally come in two sizes; .68 and .43 caliber.  .68 caliber paintballs are the standard, fit most markers and commonly used at commercial paintball fields.  .43 caliber paintballs are a smaller size and mostly used in certain paintball pistols, grenades and few rifles.  Some paintballs also come in different gradients to cater to your type of play; tournament grade paintballs cost more, however are extremely reliable while recreational grades cost less but may not be as accurate.