Ionic bonds
While there is an uneven sharing of bonding electrons in polar covalent bonds, an even larger difference in electronegativity gives rise to ionic bonds.
Ionic bonds are usually (but not always) formed between a metal and non-metal with a large difference in electronegativity, eg Sodium Chloride.
Chlorine has a far larger electronegativity and so pulls the bonding electrons towards itself completely, thus gaining an electron and forming a negative ion.
Sodium, due to its low electronegativity, will lose its bonding electron to chlorine and form a positive ion.
The ionic bond is the electrostatic force of attraction between a metal ion and a non-metal ion. Ions are arranged into a three-dimensional ionic lattice of positive and negative ions.
Ionic formulae can be written giving the simplest ratio of each type of ion in the substance. Although Sodium Chloride forms an ionic lattice, the ratio of Sodium to Chlorine is 1:1. This is why the formula is written as NaCl.