what is the difference between secondary index and non-clustered index

Question

I'm studying about indexes in a database and I'm having some ambiguity. In a non-clustered index, we learned that the datafiles are not sorted and that each index entry points to a record in the datafile. However, the secondary index doesn't seem to be any different from this one. Can the two be considered the same? If not, I'd appreciate it if you could let me know which one is different.