θέλω in the past tense

I am learning modern Greek. I'm from Poland. Could you tell me what is the difference between:

ήθελα (Imperfect)

and

θέλησα (aorist)?

Both words mean "I wanted". So, is there any difference between them? Is there any most favourite form of the verb in the past?

ήθελα = I was wanting
θέλησα = I wanted

But we prefer ήθελα in most cases....
Feel free to keep asking whatever you need