What is a chemical bond formed when atoms share electrons

If atoms share two, three or more pairs of electrons ... they are covalent bonds ... for example, methane

if the atoms share only a lone pair of electron contributed by only one of the combining atoms but shared by both ... then it is a coordinate bond ... for example, ammonium ion

The valence electrons can either be transferred between atoms or shared. When electrons are transferred from one atom to another, an ionic bond is formed. When two atoms share valence electrons, a covalent bond is formed.