Using Workflows
Workflows are linked to work item types. You can specify the workflow that is linked to a particular work item type on the Admin/Work Item Types page, as shown below:
In the above screenshot you can see that we have selected the "Simple Workflow" for the "Project" work item type. Anytime we create a new "Project" type work item it will use the Simple Workflow by default.
Changing Workflows
The workflow linked to a specific work item can be changed when creating or editing a work item: