SSIS stands for SQL Server Integration Services. It is the new data transformation standard for SQL Server 2005 and has replaced the old SQL Server Data Transformation Services. Integration Services has been literally re-written from the ground up, so it is not the same thing as DTS for SQL Server 2000 and earlier. Please be aware of that when you post your questions.
DTS packages from SQL Server 2000 can be used with SQL Server 2005, however a seperate tool must be installed to encompass that functionality. Also, in the SQL Server Management Studio, any DTS related information can be found in the Object Browser under Management -> Legacy.
The two main differences you need to be aware of with SSIS is that ActiveX has been re-written and Dynamic Properties is no longer supported. If you do a direct upgrade of your DTS package from SQL Server 2000, any Dynamic Properties tasks will be ported over as stubs (non-working programs) and your ActiveX scripts will not work and need to be re-written.
NOTE: As of this writing, I don't know if a simple cut-n-paste of an ActiveX script from a 2000 task to a 2005 task will work or if there are actual syntax changes. I will edit this FAQ as soon as I do know.
Lastly, if you have questions about your SQL Server 2000 (or earlier) Data Transformation Services package / jobs / programming, please ask the questions in forum961 which is the proper forum for it. If you are asking questions related to Integration Services, or upgrading to Integration Services, feel free to ask the question here.
Remember, the more details you post, the easier it is for us to answer your question. Also be aware that SSIS is fairly new, so all of us are still learning. Thank you for your patience.