What is the difference between "Place an order" and "Quick transaction"?
Use "Quick transaction" if you want to buy BTC/LTC instantly, just indicate the amount you are interested in and the system will automatically choose the best offers for you.
Limited offer "Place an order" enables you to create your own offer and indicate both the amount and the exchange rate. This type of transaction can only be finalized by another User. Additionally, with a limited order you can use:
a. hidden - offer won't be visible in order book
b. post only - offer has to be added to order book and if completed the maker fee for a transaction will be charged.
The same model applies for selling.