Ca2+ and Cl- are examples of ions. There are two types of ions: (1) the positively charged which are called cations and (2) negatively charged which are more commonly referred to as anions.
The charges of the ions identify their reactions with other ions. Like charges do not react and repel each other while unlike charges will attract. Hence, in this item, the ions will attract each other. Further, the reaction between this two ions will form a formula unit, CaCl2.