Converting Between Coordinate Systems
One thing you'll find yourself doing almost constantly in Unity is converting between a number of different coordinate systems. E.g. if you want to display text over someone's head you need to convert their world position into screen coordinates. If you want to figure out where a user just clicked you may need to convert screen coordinates into world coordinates. If you want one object to fly towards another object you need to convert the second object's world position into the first object's local coordinate system.
Converting from World Position to a Transform's Local Position
Example: object A wants to know where object B is in relation to it (e.g. left, right, straight ahead).
Inside a script attached to A, we would write:
// B is to the LEFT of A
Converting from World Position to Screen Coordinates
Example: We want to draw a health bar above an entity using the GUI class.
Inside a script attached to the entity we would write:
Note: Camera.main is *usually* the camera you'll want to use. If not, you'll want a reference to the appropriate camera.
... to be continued ...