A salt is an ionic compound. Ionic compounds have electrostatic attractions between oppositely charged ions. Ca2+ and No3- are the ions that are attracting each other.
If you take the acidic version of a nitrate ion, being nitric acid (HNO3) and react it with a calcium base, (say Ca(OH)2) you get a salt, being Calcium Nitrate and water.
If you take the acidic version of a nitrate ion, being nitric acid (HNO3) and react it with a calcium base, (say Ca(OH)2) you get a salt, being Calcium Nitrate and water.