What is the difference between rails structure.sql and schema.rb

I am aware that schema.rb file is a ruby file and it get created and modified when a rake migration is run, but what about the structure.sql.

I have seen in some projects schema.rb and in others structure.sql and in some both files, where do they configure which file to create.

What exactly is the difference between the two.

Is structure.sql generated is specific to a particular DB.