I am trying to figure out what is the exact difference between a document management system and archives management system? For example, what is the difference between Alfresco and Archivesspace (http://www.archivesspace.org/)?
Can Alfresco function as an archives management tool? What is the difference between the two? I read there is a record management module in Alfresco, is this what is meant by archives management?
Can Alfresco be used as an Archives Management System? Yes, of course. One real world example of this is the New York Philharmonic. They digitized their musical scores and associated artifacts going back to 1842 and then made them available online for researchers. Here is a video about it.
At its heart, Alfresco is a repository that allows you to capture any type of file, secure those files, route those files through workflows, search across the files, and associate metadata with each file. What I've just described are what most people would consider the basic set of functionality present in any worthwhile document management system.
Now, what makes that specific to archival purposes? I'm not an archivist. That's a highly-specialized field. One thing that is missing from my list of functionality above is "capture" or how the artifacts you are archiving will get into the system. This depends on exactly what it is you are archiving. One might use document scanners or high-end photography equipment, for example. None of that is addressed by Alfresco. You'll have to use third-party hardware and software and then integrate it, although many integrations exist between Alfresco and third-party capture vendors.
So I would say, yes, Alfresco can be used for archives management. But perhaps more importantly than considering whether or not a piece of software can be given a label, you should be thinking about how your users will use the software and what it is they need to get done. Then focus on how each of the packages you are evaluating can be used to achieve those goals to try to figure out whether or not each package will be a fit.