Microsoft SQL Server Integration Services (SSIS) is a platform for building enterprise-level data integration and data transformations solutions.

You use Integration Services to solve complex business problems by copying or downloading files, sending e-mail messages in response to events, updating data warehouses, cleaning and mining data, and managing SQL Server objects and data. The packages can work alone or in concert with other packages to address complex business needs. Integration Services can extract and transform data from a wide variety of sources such as XML data files, flat files, and relational data sources, and then load the data into one or more destinations.

These types of operations are commonly referred to in the industry as ETL, which stands for:
E​xtract, T​ransform and L​oad.

Integration Services includes a rich set of built-in tasks and transformations; tools for constructing packages; and the Integration Services service for running and managing packages. You can use the graphical Integration Services tools to create solutions without writing a single line of code; or you can program the extensive Integration Services object model to create packages programmatically and code custom tasks and other package objects.

The precursor to SSIS was released with SQL Server 2000, and was called Data Transformation Services (DTS). SSIS was introduced with SQL Server 2005, and was a complete overhaul of the component, centering around control-flow-centric DTSX packages. There were minor improvements with SQL Server 2008; the current version (released with SQL Server 2012) contains many differences from the 2008 one, including changes to deployment, debugging, troubleshooting and other areas. It has also had a visual face lift, with more attention to ease of development.

References:

SQL Server Integration Services (SSIS) 2012 on MSDN

Wikipedia page about the SSIS