well in simple terms , let us take an example of a DC motor having the ratings of "12v" voltage and "500 mA" current.
So you need that amount of voltage and current for satisfactory operation of your motor , thats why they are called RATED VOLTAGE and RATED CURRENT.
Now if you have a supply with the rated voltage i.e. 12V but its output current is merely 20mA , your motor wont work , so in this case you need a CURRENT DRIVER to improve the available current.
Vice-versa is the case for VOLTAGE DRIVER , its used to bring the voltage level to rated value.
I hope now its clear ......
Hope it helped !!!!!!!!