Ions are charged particles whereas atoms are neutral. When an atom looses or gains an electron it is said to be an ion. For example sodium metal in neutral state is composed of atoms where as sodium in sodium chloride is an ion.
The two ways to excite an atom are though heat and the other is through light. Both ways provide energy to atom above the activation level such that atoms become excited.
Positive ions are called cations while negative ions are called anions. The elements of group of 1 A to group 3 A of the periodic table form cations because they usually loose their electrons while the elements of group 5A to 7A forms anions because they in general set up on gaining electrons.