A protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.
 Protecting groups are used in synthesis   to temporarily mask the characteristic chemistry of a functional group   because it interferes with another reaction. 
i suggest you to view the following link for more information on protecting groups , it is mentioned very clearly.
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch17/ch17-3-4-3.html
 
Please approve !


						

							
dear chirrag ,
i m explaning this with the help of an example  
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CH0 - CH2 - C - CH3          =>         CHO - CH2 - CH(OH )- CH3 ,
if we want to carry out this conversion then the  simplest way is to protect aldehyde group & then reduction of ketonic group will give  desired product ...
 
for this , first step is protection of aldehyde group which is done by reaction with glycol (.CHOHCHOH) ..
  product of this reaction is acetal , then by LiAlH4 this c=0 group is reduced to alcohol n finally
by using few drops of acid , acetal again converts to aldehyde ...
 
						

							
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction. It plays an important role in multistep organic synthesis.