If a user wants to give some privileges to 
another user or a group of users then they will create role 
and they give this role to the users.let the privileges 
be 'insert' and 'update' on 'emp' table then instead of 
specifying them by their names they will create role which 
inturn includes these methods. this role is further given 
to the users.
            If in futher if he wants to remove some 
privilege given to the users, he can just alter the role 
and remove that particular privilege such as he can remove 
update from this created role..
           Later the users cant do updations on that emp 
table.but still can insert the values into that table.
          Hence Roles are the easiest way to grant and 
manage common privileges needed by different groups of 
database users.Creating roles and assign each role to group 
of users will simplify the job of assigning privileges to 
individual users.

roles are a set of privilages that are been set to each 
user profiles. v can create a default role and use the same 
to a number of users.