The chemical bonds which are formed by sharing valence electrons between two or more non-metals only are called covalent bonds.
Covalent bonds can be divided into three types based on how many pairs of electrons are being shared between the atoms -
SINGLE COVALENT BOND - The covalent bonds in which only 1 pair of valence electrons are shared between atleast two atoms are known as single covalent bonds.
So, in the image above you can see a single pair of electrons being shared between two hydrogen atoms.
Double covalent bonds - The covalent bonds in which two pairs of valence electrons are shared between atleast two atoms are known as double covalent bond.
In this image don't get confused by the two shells. Only focus on the valence shell ie. the last shell and you will be able to find out that three pairs of valence electrons are being shared.
Triple covalent bonds - The covalent bonds in which three pairs of valence electrons are shared between atleast two atoms are known as triple covalent bond.
In this image too, focus only on the last shell and you will be able to see three pairs of electrons being shared between two nitrogen atoms.
The modern covalent bond is conceived to be the result of the SHARING of electron density between adjacent positively charged nuclei...
...the point at which electrostatic attraction between the intervening electron cloud and the positively charged nuclei is MAXIMIZED, and the internuclear repulsion between like charges is MINIMIZED is the equilibrium covalent bond-length...
On the other hand, ionic bonding results from electrostatic interaction between positively charged, and negatively charged ions. In the ionic lattice, there are layers and lattices of interpenetrating positive and negative ions. Certainly, the particles of like charges repel each other...however, if you sum up ALL the attractive and repulsive interactions, the which may certainly be done quantitatively, a net attractive force results, and this is the so-called #"lattice enthalpy"# of an ionic salt.