Aggregator transformation is used to perform aggregate calculations such as sum,average,max min.Its an active transformation which is used to convert detailed values to summary values.If you compare with Expression transformation then the difference is that in the Expression transformation calculations are done by row by row whereas in Aggregator calculations are done for group. The Integration Service performs aggregate calculations as it reads, and stores necessary data group and row data in an aggregate cache.
The Aggregator Transformation consists of below four components
1) Group by ports
2) Aggregate Expression
3) Stored inputs
4) Aggregate Cache
You can read more about Aggregator in my below posts