There are three ways to fire up a Workflow
Run on-demand - allows a Workflow to be run by a user from the Console, or via an API.
Always On - enables a Workflow to be run continuously until it is explicitly stopped. The first step in these Workflows should be a Trigger Node
API - have third party apps or services fire up the Workflow
These options are set from the Workflow Design Settings Pane.
To allow on-demand and always-on modes, toggle the Allow Run On-Demand and Can Run Always toggles respectively from the Settings Pane.
To allow a Workflow to be invoked via API, set the properties under the API Binding band in the Settings Pane. Read more about invoking Workflows via API.