Well the NAV is the intrinsic value, but the market value would be whatever someone would be willing to pay for it.
So for a simple example, lets say that a property trust has only a single property worth $10m. Say the property is 50% geared (liabilities of $5m) and there are 4m units on issue.
NAV would be: (Assets - Liabilities)/Number of units
NAV = (10-5)/4 = $1.25
For an unlisted property, if you redeemed the units with the property fund at NAV, you would receive $1.25 per unit.
Now assume that the property is listed, the REIT would trade at the market price. If someone is willing to buy and another person sells the units in the REIT for $1.30 that would be the market price. Which would mean that the units are trading at a premium to the NAV.