Creating a Workflow for an item will make that Workflow accessible from any record of that item type. If the setting for Attach on Creation is turned on, then any subsequently created records will automatically attach to the workflow.
If the record existed before the workflow was created, then it will need to be manually attached to that Workflow. Select the Workflow button on a record, then Attach to Workflow and then select the Workflow that it should be attached to. When a record is attached to a Workflow, a new tab for Workflow History becomes available on the record (near the Related Items tab). This history will log any Workflows actions taken throughout the life of the record as well as showing what nodes are currently active. This is helpful for diagnosing issues with your Workflow as well as seeing the history of how the process flowed and what dates things happened on.