What are ratio decidendi and obiter dicta? Which part is considers as a Precedent? Thanks.

Any Judgment contains two parts i.e Ratio Decidendi and the other one is Obiter Dicta. 

Ratio decidendi is the part of ratio/law laid down in the Judgment to decide the case in hand, which is binding on its sub-ordinate courts.

Obiter Dicta is the part which contains the other discussion which helps to arrive the law to be laid down in the Judgment.

Therefore the "Ratio Decidendi" part is considered to be a Precedent and is binding on its sub-ordinate courts. My 2 cents. Thanks.