Limit is a method you can choose when you wish to buy/sell Bitcoin with your own price. This may result to pending orders. On the contrary, Market (Instant) is a method to buy/sell Bitcoin instantly using the best price listed in Order Book.
If you are using the Limit method, you can trade Bitcoin for free. But if you are trading using the Market (Instant) method, you will be charged 0.3% for every transaction.