Analytical_Function_intro1
http://www.cs.utexas.edu/users/cannata/dbms/Analytic%20Functions%20in%20Oracle%208i%20and%209i.htm

http://www.tutorialnine.com/oracle_sql/sql_functions/var_pop_function.php

Analytical functions in any DBMS, tender solutions to SQL queries to compute an aggregated value  based of on a group like "Dept No" or "Job" in a table. Obviously, these operations are used to segregate/filters/bundles the data into sectors/classes/groups/clusters. To oversimplify these operations, we may understand these operations would use  the data obtained from aggregated SQL queries, for further grouping or segregations.  The circles in the image below represent aggregated functions, and the colored sectors represent the analytical treatments or operations. In the current context analytical functions are executed after aggregation functions. 

Oracle provides many analytic functions like AVG, 
CORR,COVAR_POP, COVAR_SAMP, COUNT, CUME_DIST,DENSE_RANK, 
FIRST, FIRST_VALUE, LAG, LAST,LAST_VALUE, LEAD, MAX, 
MIN, NTILE, PERCENT_RANK,PERCENTILE_CONT, 
PERCENTILE_DISC, RANK,RATIO_TO_REPORT, STDDEV, 
STDDEV_POP, STDDEV_SAMP,SUM, VAR_POP, VAR_SAMP, 
VARIANCE.

 
 
SELECT ename,empno, sal,round( AVG(sal) over (partition BY deptno order by deptno), 2) AGVSAL, deptno FROM emp