For those using linked servers or SSIS/DTS to external databases, this MSDN article provides details on data type mappings.
An OLE DB provider exposes the data types of its data according to OLE DB type identifiers called DBTYPEs. Data types are converted between OLE DB data types and SQL Server system data types by mapping data:
From OLE DB data types to SQL Server system data types. This conversion occurs when SQL Server reads data from the OLE DB data source, either in SELECT statements or in the reading side of UPDATE, INSERT, or DELETE statements.
From SQL Server system data types to OLE DB data types. This conversion occurs when SQL Server writes data, mostly in INSERT or UPDATE statements, into the OLE DB data source in which the modified table is a remote table.