Is SSIS an ETL tool?

Yes, it is ETL Tool.

SSIS Stands for SQL Server Integration Services which is built by Microsoft. Microsoft offers MSBI(Microsoft Business Intelligence) capability and SSIS is part of MSBI along with other two SSRS(SQL Server Reporting Services) & SSAS(SQL Server Analysis Services).

Through SSIS we can perform various operation like fetching data from different data sources and clean it(transform to desired format) and load to different data sources.