1 paragraph overview: 
Alfresco uses jBoss' business process module(jBPM).  jBPM uses  jPDL (process definition language) to define a workflow in XML.  A workflow is like a state diagram.   Tasks may be in states like "awaiting approval" or "published".  Tasks transition from state to state when someone in a role ("swimlane" -- what a name) takes an action, like clicking an approve button.  jBPL provides  ways to hook up code with transitions and states, e.g. "send an email notification when this is approved".  Here's  a picture of a sales workflow example .

GUI tool 

If you don't care to  write  process definition xml by hand,  jBoss has a process design GUI available as an Eclipse plugin.   Here's a demo videoof using the GUI tool.

Tutorial:  [Getting started: how to make your first process definition. |http://docs.jboss.com/jbpm/v3/gpd/] , all the way through testing..  Downloads for tutorial: -[-http://www.jboss.com/products/jbpm/downloads-|http://www.jboss.com/products/jbpm/downloads]--[|http://www.jboss.com/products/jbpm/downloads]--[|http://www.jboss.com/products/jbpm/downloads]--[|http://www.jboss.com/products/jbpm/downloads]--[-jBPM downloads-|http://www.jboss.com/products/jbpm/downloads]--, jBPM starter kit is supposed to have everything needed for the tutorial.- _Installation instructions have drifted from products e.g._\[ a jboss blog:\|http://blogs.jboss.com/blog/kaers/\] : 

AF also supplies a Getting Started bundle.  Use this one instead; the readme.txt gives steps to point Eclipse to the jBPM runtime .

Links: 

Alfresco: overview page on the Wiki; has pointers to useful looking guides, including  Advanced Workflows - rolling your own workflow .

jBPM jump page : docs, downloads, fora, etc.

jBoss JBPM documentation . The jBPM User Guide picks up where the quick start tutorial leaves off.