Alternate data streams look like normal files but with an appended identifier.
For example test.txt:stream1 is an alternate data stream of test.txt. Move or copy the file and the ADS goes with it.
They can be created like other files (“echo > test.txt:stream1”)
You can see them with “dir /r” at the command line.
You can even have an alternate data stream with no corresponding file. In my opinion this is what thumbs.db should have been.