Reduction Reaction : Reduction in chemistry can be defined in two ways 
1.Addition of electrons 
2.Addition of hydrogen atoms ( organic view)
in reduction reaction the negative charge on the atom being reduced increases .
Reducing agent: It is a molecule which provides pair of electrons or H-atoms for example Na atom can act as reducing agent becoz it can donate one electron .
(ii) Hydrogen gas is also used as reducing agent in combination with a metal atom ( generally transition metals 
Note: ANY MOLECULE HAVING ANY NUMBER OF HYDROGEN CAN POSSIBLY ACT AS REDUCING AGENT Example : Hydrogen gas , NaBH4 , H2O , LiAlH4 etc..!

WHICH MOLECULES UNDERGO REDUCTION :
1. The molecule or atoms having high positive charge on it for ex: Fe(iii) is easily reduced due to higher positive charge .
2. In organic terms , Molecules containing multiple bonds can be good reactants for REduction for ex: Alkene C==C , alkyne having C-C tripple bond , Carbony compounds C==O and compounds having C==N .
SO in general any molecule having multiple bond can be reduced . Few examples are shown below