WHETHER / IF

• Whether 
• more formal
• Afeter a  preposition
• two choices
• I don't  know  whether  you  like  coffee or tea  (only  two  possibilities)
• I don't know  if you  like  coffee or tea (this,  that  or anytihing  else)
• To  know  more http://www.bbc.co.uk/learningenglish/english/course/upper-intermediate/unit-3/session-1