ADS is a utility to help you finding, inspecting and (optionally) removing Alternate Data Streams from your NTFS disks.

In a NTFS volume a file may contain more that one data stream. From Explorer and most programs the user can see only the default, unnamed stream, but other may be present for useful (or harmful purposes).

In Vista, but not in XP, you can open a DOS box and use dir /R to display the alternate data streams whose name is displayed in the form:
file_name:stream_name:$DATA

Depending on your Windows version some DOS commands and redirection are ADS-aware, as well as Notepad and maybe other programs (including JPSofts 4NT and Take Command). For instance, you can dump the ADS contents by using
more < file:stream
(notice that you do not add the final :$DATA).