a adjustment is some thing that is written off on a invoice.eg a customer short pays a 100$ invoice by 5 $ you write off this 5$ this is an adjustment.
while a discount is something a customer gets from our side. eg if the customer has net 30 5 discount in his payment term this means if he pays within 30 days he gets a discount of 5 so for the above eg the customer pays 95$ within 30 and oracle gives him a discount of 5$ automatically
from accounting perspective the 5$ of the adj will go to the account of the receivable activity