ionic bond
Instead of sharing, sometimes one atom simply gives an electron away and the other takes it. Now one is positively charged and the other negatively charged, and opposite charges attract — like a balloon rubbed on hair sticking to a wall. That electrostatic grip between oppositely charged ions is an ionic bond.
Precisely, an ionic bond is the attraction holding together ions of opposite charge, formed when one atom transfers one or more electrons to another. A metal that loses electrons becomes a positive cation; a nonmetal that gains them becomes a negative anion; the two pull together. In a solid, this is not one isolated bond but a vast repeating lattice in which every ion is surrounded by neighbors of the opposite charge.
Ionic bonding explains the properties of salts: they are hard but brittle, melt at high temperatures, and conduct electricity only when melted or dissolved, once the ions are free to move. The honest caveat is that perfectly ionic bonds are an idealization — even in 'ionic' compounds the electron transfer is never 100% complete, so real bonds always carry some covalent character.
In table salt (NaCl), each sodium atom gives its single outer electron to a chlorine atom. Sodium becomes Na⁺, chlorine becomes Cl⁻, and they lock into a cubic lattice. Because it takes so much energy to break that whole grid of attractions, salt does not melt until about 800 °C.
Salt's high melting point comes from a whole lattice of ionic attractions.
An ionic 'molecule' like NaCl does not really exist as a separate pair in a crystal. The formula NaCl gives the ratio of ions, not a discrete two-atom unit; each Na⁺ in a salt crystal is bonded equally to six Cl⁻ neighbors, and vice versa.