Q:

Does Alfresco support monitoring entire folder tree for changes? 

I am interested in triggering code to run when any changes are made to any documents or folders under a specific folder in alfresco.  I am sure alfresco has to support this some how but I am not entirely sure what the feature/api/service is called or what to google for.  What has this ability in alfresco and what is it called?  
I am aware that its possible to create custom content rules in javascript and then add them to a folder to be triggered by some action.  I did that a couple years back for auto filing documents that were dropped into a specific folder.  Problem is that requires adding that action to each folder you want to monitor.  I want to monitory an entire directory tree basically. 
EDIT:
Correct me if I am wrong, but it appears that Events API may do what I need to do.  I found this right after posting my question. 

A:

You probably just want to create a Rule on the top folder, and set it to apply to subfolders.  If you're interested in changes, then set a When of "Items are updated".  Depending on your needs, you could also restrict it to only items of a given type or aspect (eg to pick up document changes, but not folder changes)
Finally, you can either have your Rule run a JavaScript file (Execute script) to perform the business logic, or you can register a Java Custom Action and have that triggered

A:

@Gragravarr answer is the best for handling events over a subtree of the repository.  
In case what you need to monitor is only a subset of all the content of the subree (e.g. watch all the incoming documents, but only if they are PDF) and possibly scattered across the whole repository, then creating a specific content type and handle events using Policies is probably a better option.