This lesson goes over how to assign oxidation numbers (oxidation states) to elements.
Redox Reactions: Crash Course Chemistry #10
All the magic that we know is in the transfer of electrons. Reduction (gaining electrons) and oxidation (the loss of electrons) combine to form Redox chemistry, which contains the majority of chemical reactions. As electrons jump from atom to atom, they carry energy with them, and that transfer of energy is what makes all life on earth possible.