Covalent or ionic: transfer of electrons
covalent or ionic: sharing of electrons

When two atoms react, they form either of two kinds of bond, ionic bonds or covalent bonds.

Ionic bonds are the type of bonds where there is transfer of electrons from one atom to another.  The electrons are removed and from one atom and attached to  another. A good example is salt which is composed of sodium and chlorine. Sodium readily loses one of its electrons and chlorine readily accepts it. Before losing the electron, sodium has a positive charge, but then becomes negatively charged after giving up the electron. Chlorine has a positive charge before gaining the electron but becomes negatively charged after gaining the electron. These opposite charges between sodium and chlorine attract the two elements together to form the ionic bond.

Covalent bonds are the kind of bonds formed when two atoms share electrons. Here there is sharing, none of the atoms loses an electron and none gains. A good example is water which is formed when oxygen  shares two electrons, each with an atom of hydrogen.

 The Oxygen atom forms two covalent bonds with the pair of hydrogen atoms.