In a neutral atom - that is, one showing no external electrical effects - the total quanitity of electricity or charge represented by its electrons is balanced by an equal quantity associated with the atomic nucleus - balanced because there are two types of electricity, the kind represented by the electrons being termed negative and that associated with the nucleus being called positive. This difference is called polarity.
In many materials, particularly metals, it is possible to disslodge one electron from an atom with reletave ease. When an atom loses an electron its electrical balance is upset. The electron removed is a free particle of negative electricity, and since the atom has lost that amount of negative charge it is left with an equal excess of positive charge. The free electron is called a negative ion and the atom lacking an electron is called a positive ion. When some billions of similar ions are collected together the quantity of charge becomes large enough to cause observable effects.
More commomly associated with object of those post-thieving debates around which one tries to denounce the act of his crime to the court and police (aka. the popo).