If we use Aggregator without using group by option we will 
get only one row from source. It is a default charater of 
Informatica.

e.g If there are 20  rows in the source table and only 
using SQ and followed by Aggregator then target table then 
the target table will be populated by the last row coming 
from teh source table.

if u use aggregator t/r without using group by function u
get  one summirized result but group by option allows u get
deatil result.
for example .without using groupby u want see a releince
store sales .u will get sales of whole store but when uuse
group by u get imtemized sales result
storecode   itemname       sales 
101          vegetable      500
101          vegetables     700
101          electronics    5000
101          electronics    10000
101          cooldrinks      200
101          cooldrinks      500

result without groupby: store sales:17400
result with groupby: vegetables sales :1200
                     electronics sales: 15000
                     cooldrinks sales: 700