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.