This occurs in metals and is when electrons are given out from the metal atoms to make a "sea" of free electrons in between all of the metal atoms. These free electrons hold the metal as a mass together. This is known as being a giant structure.
The free electrons present make it possible for electricity to be transferred easily because the charge is carried by them. Because the metal atoms have lost electrons, they have a positive charge.
This occurs typically between a metal and a non-metal (from groups 1 + 2 and 6 + 7 on the periodic table). The metal atom loses electrons which are taken by the non-metal. Therefore, the metal ion produced has a positive charge and the non-metal ion is negatively charged.
Sodium (metal - group 1) + Chlorine (non-metal - group 7) ----> Sodium Chloride (NaCl)
In the example below, the sodium atom has lost an electron to form a positive ion. The chlorine atom has gained the electron lost from the sodium to form a negative ion. The NaCl molecules are arranged in a giant structure to form the substance more commonly known as Salt.
This is when electrons are shared between atoms. Covalent bonding occurs between non-metals due to the fact that all of the atoms need to gain electrons, so they have to share.
Common substances that covalent bonding occurs in: Water (H2O), Hydrogen gas (H2), and Methane (CH4)