Chemical bonding is the major part of chemistry which is an interaction between two or more atoms that holds them together by reducing the potential energy of their electrons. In other words Bonds are the like chemists "glue" - which hold atoms together in molecules or ions. Valence electrons are the outer shell electrons of an atom which take part in chemical bonding.
Atoms gain or lose electrons to attain a more stable noble gas - like electron configuration (octet rule). There are two ways in which atoms can share electrons to satisfy the octet rule:
Ionic Bonding - occurs when two or more ions combine to form an electrically-neutral compound
The positive cation "loses" an electron (or 2 or 3)
The negative anion "gains" the electron (or 2 or 3)
The anion steals the electrons from the cation.
Covalent Bonding - occurs when two or more atoms combine to form an electrically-neutral compound.
The electrons are shared between the two atoms.
Both atoms don't have charge in the beginning and the compound remains with zero charge.
The chemical activity of an atom is determined by the number of electrons in its valence shell. With the help of concept of chemical bonding one can define the structure of a compound and is used in many industries for manufacturing products.