Ionic bonds are a type of electrostatic bond between two atoms that is weaker than covalent bonds, but usually stronger than hydrogen bonds or the van der Waals force, which is what holds water to itself. They are formed from the mutual attraction of ions. Typical participants in an ionic bond are a metal and a non-metal, such as sodium and chlorine. Sodium and chlorine combine to create a typical ionically bonded molecule, sodium chloride, or common table salt.
Ionic bonds can be explained using the theory of electron orbitals. Every atom has a number of electron orbitals equal to or greater than one. Each orbital has a maximum electron limit, after which a new orbital is created. The number of orbitals in the elements we know ranges from one for hydrogen or helium, to six, seven, or eight for the larger molecules with atomic numbers greater than uranium.
Electron orbitals "want" to have the maximum number of electrons. That's when they're at their lowest, or most stable energy state. When an atom missing only a single electron from its top orbital comes into contact with an atom that only has one electron in its top orbital, the "wanting" atom "steals" an electron from the other atom, thereby stabilizing its orbital. The consequence is that it now has one more electron than it has protons, making it negatively charged. The "victim" of the stolen electron correspondingly gains a positive charge. In electromagnetic theory, opposites attract, so the atoms are forced to hang around each other until they get broken up by, say, heat. That is what ionic bonds are.
Because the electron orbitals slightly overlap in ionic bonds, they are thought of as weakly covalent, or bonded by shared electrons. The strongest bonds are highly covalent, with electron shells deeply overlapping each other. Diamond is an example. The atomic difference between ionic bonds and covalent bonds is what makes the melting point of diamond so much higher than that of rock salt. Sometimes the electron difference between the ions in ionic bonds is greater than one. The greater the electric polarity difference, the stronger the bond is.
Immersion in water usually knocks a ionic bonded material around enough that it dissolves. The molecular nature of ionic bonded materials also makes them prone to arranging into crystals. They are horrible conductors of electricity, unless molten or suspended in a solution.