The nature of the electrostatic charge causes objects to be either repelled from each other or attracted to each other. If they have the opposite charge, for instance if one atom is negatively charged and another is positively charged, the atoms are attracted to one another. The rule that states this is "opposites attract." If two atoms have the same charge, they repel each other. An atom's charge is based upon the number of electrons and protons which it posseses.
How do you know it exists?
I know that the electrostatic charge exists because of the fact that in our lab I could see the rubbed straw attract the uncharged paper scraps. Also, the electroscope allowed me to see static electricty in action by the way that the foil was attracted to the foam pad and plate, both charged objects.
How do you know that two types of charge exist?
I know that two types of change exist because objects would always repel each other if their charges were always the same. Objects of the same charge repel each other and objects with opposite charges attract each other. Remember, opposites attract!