To get the next value of a sequence, we use nextval.


 NextVal is pseudo column, which gives you the AVAILABLE 
number in the sequence.
 Remember the nextval value also depends on the cache you 
have given, while creating the sequence.
 If you want current value of a sequence you can use CurVal.
 In a session you cannot use current value without getting 
next value.
 In 11g the you can use the nextval without using the query.
ie 
   select seq_name.nextval into ln_seqnextval from dual;
           can be replaced by 
    ln_seqnextval := seq_name.nextval;

NextVal is pseudo column, which gives you the unique number 
in the sequence.
nextval increment the sequence and return new value.
currval can only be accessed in given session after at least
 one nextval has been issued.