When you place an order on Vaultoro you may place two types of orders:
A limit order
A limit order is an order to buy gold at no more than a specific price, or to sell gold at no less than a specific price (called "or better" for either direction). This gives the trader (user) control over the price at which the trade is executed; however, the order may never be executed ("filled") if the price never reaches the limit because it was set for lower than the current price on the buy side, and higher than the current sell price on the sell side.
Limit orders are used when the trader wishes to control price rather than certainty of execution.
A buy limit order can only be executed at the limit price or lower. For example, if an investor wants to buy gold, but doesn't want to pay more than 0.1BTC for it, the investor can place a limit order to buy the gold at 0.1BTC. By entering a limit order rather than a market order, the investor will not buy the stock at a higher price but may get less gold than he wants or not get any gold at all.
A sell limit order is analogous; it can only be executed at the limit price or higher.
A market order
A market order is a buy or sell order to be executed immediately at current market prices. As long as there are willing sellers and buyers, market orders are filled. Market orders are therefore used when certainty of execution is a priority over price of execution.
A market order is the simplest of the order types. This order type does not allow any control over the price received. The order is filled at the best price available at the relevant time. In fast-moving markets, the price paid or received may be quite different from the last price quoted before the order was entered.
A market order may be split across multiple participants on the other side of the transaction, resulting in different prices for some of the shares.