what is the difference between biztalk and SQL DTS?

SQL DTS is a tool for importing and exporting data from and to Microsofts SQL Server.

BizTalk server connects two sepparate systems with eachother by defining the messages (EDI/XML/FLATFILE/ETC) used by these systems and connecting these messages with eachother.

For more information look at the Microsoft website, it's got loads of information about these subjects.