According to the U.S. Army, having a purpose gives those you're leading "a reason to act in order to achieve a desired outcome," while "providing clear direction involves communicating how to accomplish a mission: prioritizing tasks, assigning responsibility for completion, and ensuring subordinates understand the standard." Finally, "motivation supplies the will to do what is necessary to accomplish a mission."
The Army follows a BE-KNOW-DO plan:
Leadership begins with what the leader must BE, the values and attributes that shape a leader’s character. Your skills are those things you KNOW how to do, your competence in everything from the technical side of your job to the people skills a leader requires. But character and knowledge—while absolutely necessary—are not enough. You cannot be effective, you cannot be a leader, until you apply what you know, until you act and DO what you must.
[For a PDF of the full Army leadership manual, click here. Thanks, Bob!]