Market – If you put in a “market” order, or “at the market”, you are either placing an order to BUY at the ASK price, or placing an order to SELL at the BID price.  My opinion is that one should ONLY place a market order to limit a loss.  If you miss a buying opportunity, that is OK, there will be more.

Limit – If you place a “limit” order, you are telling the market what price you will either buy or sell at.  In my opinion, this is how every order should be placed.  I have learned the hard way that the market, or market makers, will place their interests above yours if they have the opportunity.

Fill-or-Kill – This is a limit order that you do not want to remain open for any length of time.  Either the market fills your entire order at your price, or it is cancelled.

Market on Open – This means you will have a “market” order for the opening price.

Market on Close – This means you will have a “market” order at the closing price.

Day Orders – This is a time limit that is on your “limit” order.  If the order does not get processed today, it will be cancelled.

Good till cancelled orders – This is a limit order that will remain open UNTIL you cancel it or it gets filled.  Many brokers put a 60 day limit on these orders.

Stop Loss Orders – This is a protective order.  If you purchase a stock for $10, and you have placed a limit on your losses of 10% for any position, you can place a “stop loss” order for the position at $9.  If the stock price reaches $9, you will automatically sell you position.  IF the stock GAPS down below $9, you may get a much lower price than $9.  There are other, better, ways to limit your losses.