Ducts are tubes that permit communcation between cavities.  Ducts allow the paranasal sinuses to drain into the nasal cavities through ostia.  The following diagram shows the structures that you should be aware of.
 

The frontonasal duct drains the frontal sinus.  The ostium leads into the front end of the middle meatus.

The maxillary ostium is the source of communication between the sinus and the nasal cavity.  It opens into the posterior aspect of the hiatus in the middle meatus or behind the lower third of the uncinate process.  The ostium is much smaller than the bony defect as the mucosa fills the area and defines the extent of the opening.

The nasolacrimal duct drains the lacrimal sac.  It runs from the lacrimal fossa in the orbit down the posterior aspect of the maxillary bone and empties into the anterior aspect of the inferior meatus.  The duct lies very close to the maxillary ostium.

The anterior ethmoid cells drain into the middle meatus via the infundibulum.  These anterior cells include the agger nasi, the bulla ethmoidalis.  The posterior cells drain into the superior meatus.

The ostium of the sphenoid sinus drains into the anterior wall of the nasal cavity approximately 1cm above the superior turbinate.  It opens into the sphenoethmoid recess.

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