Tabular Data Stream (TDS) is an application layer protocol, used to transfer data between a database server and a client. Initially designed and developed by Sybase Inc. for their Sybase SQL Server relational database engine in 1984, and later by Microsoft in Microsoft SQL Server. For more info see wikipedia at URL http://en.wikipedia.org/wiki/Tabular_Data_Stream
FreeTDS is the open source implementation of the TDS protocol described briefly in the previous paragraph.
Often you must provide a query result in CSV or other text format for integration with other systems or simply to import the data to an Excel spreadsheet for pivot tables.