Unity 3D has 4 coordinate systems: World, Screen, Viewport and UI.

The world coordinate system is left handed (as Direct X) where x positive axis is right, y positive is up and z is positive into the screen.

Screen coordinate system is bottom-up: (0,0) at bottom-left corner and (pixelWidth-1,pixelHeight-1) at right-top; x axis is positive right and y is positive up. The z position is in world units from the camera.

Viewport coordinate system is normalized and relative to the camera, so the bottom-left point is (0,0), the top-right is (1,1). The z position is in world units from the camera.

Finally UI coordinate system is top-down: the y coordinate varies from zero at the top edge of the window to a maximum at the bottom edge of the window. The upper-left point is (0,0); the bottom-right is (1,1)

There are many functions to convert between all these different coordinate systems, see

Camera.WorldToScreenPoint

Camera.WorldToViewportPoint

Camera.ScreenToViewportPoint

Camera.ScreenToWorldPoint

Camera.ViewportToScreenPoint

Camera.ViewportToWorldPoint

GUIUtility.ScreenToGUIPoint

GUIUtility.GUIToScreenPoint