Workflows begin with a definition: the set of conditions and actions used by the workflow to manage your business process. This is essentially what your workflow does.
An association is the assignment of a workflow to an object. This is where you want to run your workflow.
An instance is the act of starting the workflow. This is your running workflow.
With regards to workflow definitions, Replicator replicates the creation and modification of SharePoint Designer workflow definitions. Replicator does not replicate the creation or modification of workflow definitions in visual studio. We do, however, replicate the activation of workflow features, associations and instances of visual studio workflow definitions that have been deployed on both source and target.
SharePoint users can interact with the workflow instance and associated workflow tasks on any server, and the updates and impacts are replicated to all target servers.
When a supported workflow is started for a document or list item on a server, the workflow instance is captured and replicated to the target servers. The workflow tasks and history items associated with the workflow are neither captured nor replicated. These workflow artifacts are created on the target servers when the replicated workflow instance is initiated for the document or list item.
It is vital to note that Replicator requires that the workflows be installed and enabled on all web applications where the workflow will be processed. Many built-in workflows require that certain features be activated in order to be used.