Ionic Bonds happen between a metal and a nonmetal.  The most
common type, and the easiest to understand is sodium chloride (NaCl)
or table salt.
Sodium is an element from Group 1 of the
periodic table.  This means that it will have one
extra electron in the outer shell.  Atoms in groups
1 and 2 will be highly reactive because that one
extra electron can be lost easily.
Chlorine is from group 17 of the periodic
table.  This means that it will have one space
for an extra electron in the outer shell.  
Atoms in group 17 are highly reactive because
they will take electrons from other elements
in order to fill their outer shell.
Ionic Bonds - The atoms