Why does my dodge Dakota 2001 crank but wont start?

Could be for a number of reasons- there may be a problem with the solenoid, or the alternator (called the generator in the US) may be wearing out and not recharging the battery enough to make it start the car. Other explanations may be that the spark plugs may need changing, the plug leads could have worn out and need changing, or that there is wear in the distributor, either in it's cap or on the rotor arm. It may also be simply that the engine has become flooded (i.e. too much fuel in the combustion chambers), which just requires time to let in drain away.