Market: Buy (or sell) at the best current market price.
Limit: Buy (or sell) at a fixed price (or better).
The advantage of limit orders over market orders is that they can ensure a better average fill price, since the bids or asks used to fill the order will be at the limit (or better). You get to set the price you want to execute an order at. The disadvantage is that there's no guarantee the order will completely fill (or fill at all) - it may not be the case that adequate bids or asks to fill the order will appear on the market.
Examples: If the lowest current ask for BTC/NZD is $500, but you think you can get your buy order filled a bit lower, you might put in a limit buy order at $490.