When I first got my diesel people would always say "diesels don't have spark plugs, they have glow plugs."  Though they are correct, glow plugs and spark plugs are two totally different things which are almost totally unrelated and have completely different functions.

Spark Plugs ignite the gas and air in each cylinder and are a mandatory for a gas motor to function.

Glow Plugs are items installed into each cyl and activate when you turn the key into the on position.  When this first happens you will see a light that says "wait to start" or something similar.  This is because the glow plugs are charging each cyl with an amount of heat that allows a cold diesel engine to fire properly.  In the summer good glow plug condition isn't as necessary and in fact many diesel engines don't have them and either incorporate a method of direct injection or an either based system to allow a cold diesel to start. 

But in the winter, in glow plug equipped diesels, good glow plug condition is almost 100% mandatory.  Though my truck started every single time this winter, even when I was living in Alta and I'd come to a truck buried for days at a time under 5 feet of snow at -10 below F with no place to plug a block heater in, it would still start and I never had to use either, which can damage some diesel engines. 

Glow plugs have a relay and resistor which is also controlled by the ignition key switch.  When the key is put in the on position the relays are heated until the electrical resistance is great enough (preset in the system) to start the engine.  This is why a warm engine will only have the glow plugs turn on for a split second while a cold engine (particularly American ones) will have their glow plugs activated for up to 30 seconds or more.  Sometimes it will take you more than one glow plug run to start your truck too but if that's the case it can be considered a sign that perhaps your glow plugs are failing. 

When they do fail it's best to replace them all at the same time as they function similar to the way a truck's headlight does: if one failed chances are the others will fail shortly too. 

Make sure your glow plugs have a good connection between each one and your truck will also start easier.

And one thing I noticed is Japanese diesels seem to start a lot easier than American and European diesels.  I'm not sure what this will do but typically American and European diesels are fine with ether (starting fluid) while many Japanese diesels will be damaged.