What is the difference between 'le' and 'un' in french?

'Le', 'la', 'les' is used when you are asking or describing something in particular.

"Can I have the blue pen?" - "Est-ce que je peux avoir LE stylo bleu?" - You only want the blue pen. 

"My dad's car is pretty." - "LA voiture de mon père est jolie."

Where as using 'un' or 'une' is when you are asking or talking about something without being specific.

"Can I have a pen?" - "Est-ce que je peux avoir UN stylo?" - You don't care what pen you are being given.

"A car is useful" - "Une voiture est utile"