In reply to Viren_B:
By randc you can have cyclic randomness of a single varaible. While using unique, you can choose a unique combination from a group of variables.
randc int a; // only variable 'a' is having unique value in randomization
rand int a,b,c;
constraint a_cn {unique {a,b,c};} // the variables a,b,c all shall have unique/different values
Moreover, as stated in example, using unique keyword, you can exclude some of the values from some array (use those constrained-excluded values array along with unique).
Refer IEEE 1800-2012 Section 18.5.5 for more information.