It ensures that the ETL process functions well with other upstream and downstream processes. A Data warehouse implementation is a collection of many components, and hence integration testing is a must. Typically, system testing only includes testing within the ETL application.

It is same as ETL testing.

Key responsibilities of an ETL tester are segregated into three categories

Stage table/ SFS or MFS
Business transformation logic applied
Target table loading from stage file or table after applying a transformation. 

Some of the responsibilities of an ETL tester are:

Test ETL software
Test components of  ETL datawarehouse
Execute backend data-driven test
Create, design and execute test cases, test plans and test harness
Identify the problem and provide solutions for potential issues
Approve requirements and design specifications
Data transfers and Test flat file
Writing SQL queries3 for various scenarios like count test