SQL Server Integration Services ( SSIS ) Architecture
SSIS is a component of SQL Server 2005/2008 and is successor of DTS (Data Transformation Services) which had been in SQL Server 7.0/2000. Though from end-user perspective DTS and SSIS looks similar to each to some extent, it is not the case in actual. SSIS has been completely written from the scratch (it’s a new enterprise ETL product altogether) and hence it overcomes the several limitations of DTS.
I have written an article covering SSIS architecure and how it is different from DTS. Understanding these things will let you understand internal of SSIS or how SSIS actually works.
Refer this link for more details.