Driver vs HAL in NRF52

I am budding Embedded Engineer, currently, I would like to learn how to program nrf52dk. While I am exploring info center, Peripheral examples I have noticed that most of the peripherals has two section:

1. Driver.
2. HAL.
My questions are the following:
1. What is the difference between these two? and which one to opt
In future I am interested in implementing RTOS on nrf52. will it make any difference if i learn coding with driver or HAL?

Thanks in advance,
Ashok