Yes, diesel engines are more difficult to start. Briefly, while gasoline engine combustion is initiated by a spark from a spark plug, diesel combustion is created by the self-ignition of an air/fuel mixture introduced into the combustion chamber. The compression stroke initiated by the piston rising in the cylinder (the combustion chamber is at the top of the cylinder) will increase the temperature to 550° Celsius, which is hot enough to cause the fuel to ignite without a spark. In winter, the high temperature necessary to ignite without a spark may never be reached due to cold starting temperature and the cold temperature of both the engine and the air. Diesel fuel is not as volatile as gasoline, which makes ignition even more difficult. One remedy is to fit a glow plug into the combustion chamber. This glow plug will maintain a warmer air temperature and will keep the cylinders warm as well. This site includes separate sections on  diesel fuels and diesel engines.
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