What is the difference between liquid nitrogen and nitrogen?

well liquid nitrogen is just Super-Cooled nitrogen.

You would have to cool gaseous nitrogen to a temperature of -196 C or -321 F

Chemical difference ...None.
Physical difference...A liquid separated from liquefied air at -196°C.
Vaporises (Boils Off) to its natural gaseous state at this temperature and Atmospheric pressure. It soon gains the temperature of the atmosphere as a gas while the liquid stays at -196°C while boiling..

Very briefly, the Liquefaction of Air process requires the Compression and Cooling of Clean, dry air by a series of these processes until the air changes to liquid at its Critical Temperature and Pressure of -149°C and 35 atm.
The liquid air is then Fractionally Distilled to separate Nitrogen at -196°C and Oxygen at -183°C, at atmospheric pressure. The other gases contained in the atmosphere can also be separated in this way as required.