Dear mohini,
The -OH group  attached to the benzene ring in phenol has the effect of making the ring  much more reactive than it would otherwise be.

One of the lone pairs on the oxygen atom in the -OH group overlaps with the delocalised ring electron system . . .

. . . giving a structure rather like this:

The donation of the oxygen's lone pair into the ring system increases the electron density around the ring.
A benzene ring undergoes substitution reactions in which the ring  electrons are attacked by positive ions or the slightly positive parts  of molecules.  In other words, it undergoes electrophilic substitution.   If you increase the electron density around the ring, it becomes even  more attractive to incoming electrophiles.  That's what happens in  phenol.
 






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Sagar Singh
B.Tech IIT Delhi
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