Liquefied Natural Gas (LNG) is natural gas that through liquefaction becomes a cryogenic liquid. At that temperature, about -160°C (-262°F), the volume has been reduced 600 times. A car reduced 600 times would fit on your thumbnail. Liquefied natural gas is easier to store than the gaseous form since it takes up much less space. LNG can also be easier to transport due to the reduced size.
Liquefied natural gas is primarily composed of methane, typically about 95% but it may vary between 85 and 99%. LNG is nonflammable, odorless, colorless, non-corrosive and non-toxic because of the removal of many non-methane natural gas components. LNG composition, like natural gas and other petroleum products, will vary slightly depending on the source and the liquefaction method used. This variation can create problems for LNG users, similar to how gasoline octane differences can cause differences in engine performance.
LNG is nonflammable because of the lack of oxygen in the liquid. When LNG does vaporize however it becomes natural gas that can mix with air and become flammable. The cold temperature of the liquid and the density of the natural gas (which is buoyant) can form a white vapor cloud when spilled. The flammable cloud can be outside the visible cloud, especially when the humidity is below 55%. Natural gas has an odor added for detection/safety reasons in many cases, but LNG does not have the oder agent added (and it is removed during the liquefaction process).
Other LNG properties, like density and viscosity, will also vary slightly depending on the source of the LNG. The values in the table should be used as guidelines. For a more thorough discussion of LNG specifications and differences, one place to start is at the University of Texas Center for Energy Economics along with the Society of Automotive Engineers for vehicular specifications.