Let's say you place a buy-stop-limit-order of company XYZ.

Your stop is 80. Your limit is 100. The limit order gets activated at 80 then, if the stock reaches 100, it gets executed.

How is this different from a buy limit order of 100? The stock fluctuates up and down until it reaches 100
at which point it gets executed.

Either way, it gets executed as a limit order at 100. So what benefit does a stop-limit-order offer over a limit order? (when buying)

Can someone clarify Buy-Stop-Limit-Order? I already understand Sell-Stop-Limit-Order.

The stop-limit order gets converted into a limit order once the stop price is reached. As long as the stock price is between 80 - 100, as in your example, the stop-limit order will get executed, no once its reaches 100. 
You use stop price as a trigger, and with the limit you are specifiying how much you are willing to pay for the stock. 
I hope this helps, just opposite of the sell stop-limit order. 

You already understand that a STOP is a trigger point.  When the stock price hits the STOP price a Limit order is then immediately entered.  In this discussion that is a LIMIT order to BUY XYZ at 100.  The Limit places the upper end of what you are willing to pay.  The way to read the placed LIMIT order is "I want to buy XYZ but pay no more than 100". It is even conceivable that you might purchase it below 80 if after it hits 80 (the STOP), it then begins declining. 
One simple way you might use a BUY STOP-LIMIT order is  to prevent a stock  from running away from you.  For example, let's say you notice one day significant upward movement in stock RST. You don't want to pay that much because of the big upward spike and wait. The stock continues to zoom and all indications are that it might rocket. So now, short of abandoing your interest, you get in and pay perhaps more than if when you first noticed the upward movement, you still kept your wishful thinking, namely, "I'll buy some on a dip", but had also placed a BUY STOP-LIMIT in the event the stock continued its rapid upward movement. When the STOP is triggered, at least this way then you'll bet in at the LIMIT price or less.

giardina:

Let's say you place a buy-stop-limit-order of company XYZ.

Your stop is 80. Your limit is 100. The limit order gets activated at 80 then, if the stock reaches 100, it gets executed.

How is this different from a buy limit order of 100? The stock fluctuates up and down until it reaches 100
at which point it gets executed.

Either way, it gets executed as a limit order at 100. So what benefit does a stop-limit-order offer over a limit order? (when buying)

Can someone clarify Buy-Stop-Limit-Order? I already understand Sell-Stop-Limit-Order.

When the price of a stock reaches  its resistance, many people sell it to lock in to profits and wait to see whether the stock will break through the resistance or not. If it does, they buy it back. For example, XYZ has its resistance at $80 and you own it. To implement that strategy, you would sell your shares as close to $80 as you can (by placing a limit order, say at 79.99). Then you would place a buy-stop-limit, say with a stop at 81 and a limit whatever you feel comfortable paying for it. This way you loose $1.01 on the uptrend but reduce significantly your losses on the possibility of a strong downtrend after the stock hits its resistance. 

Can I enter a stop at 80 and a limit at 80 ?

I want to sell at 80.

WOAH! WOAH! There is one thing very fundamentally wrong that is not being understood here. Namely, the way a Limit Order works.   A Limit Order is NOT a request to buy or sell a stock at a particular price.  It is rather an order that sets the maximum or minimun at which you are willing to either buy or sell respectively a particular stock.  A Buy Limit Order of 100 on stock XYZ is an active order to buy XYZ at 100 or less, i.e, get me the best price, but I don't want to pay more than 100.  The actual buy is not executed at 100, unless of course that is the best price (from the buyer's persepctive) that can be gotten.  It might very well have been executed at 96 or 95 or 98, etc. 
You can qualify when a Limit Order become active by placing a trigger point on it, i.e. a STOP. E.G. a BUY STOP 80 LIMIT ORDER 100.  This means when the stock price becomes 80 immediately place a order for me to BUY XYZ stock at a price no greater than 100.  Hope this helps.

MRPARKLAND.  All you need to place is a SELL LIMIT ORDER at 80.  This means you will take nothing less than 80 for your shares.  If the stock were selling at 85 when you entered this order, it should be immediately executed because the execution satisfys your LIMIT, i.e., I'll take nothing less than 80.  If, on the other hand, the stock were selling at 75 when you entered this order, it would sit actively on the books and only be executed if the stock rose to 80. 
If on the other hand, you were worried about protecting profits, you would use a SELL with a STOP strategy.  Say you bought the stock at 75, it rose to 90 and now you're seeing it decline and its sitting at 85.  You hope it might rise again, so you don't want to sell while its at 85 because you believe it will rise back up.  However,  if it continues falling you also don't want to lose all your profit, so you decide you want out at 80.  You could then place either of the following two orders: 1)SELL STOP 80 - when the stock price hits 80 (the STOP), the sell immediately becomes a Market Order and will be sold.  The actual price will then more depend on the movement of the stock and the execution performance of your "broker".  2)SELL STOP 81 LIMIT 80 - when the stock price hits 81 (again the STOP), a SELL LIMIT ORDER at 80 will become active and you should get an execution price of your sell no less than 80.  Good Luck.

SELL STOP 81 LIMIT 80 is the safer way to go.

I place a sell limit order without a stop and the sell order executed at market which was less than I expected to sell

at. Live and learn I say.

If a stock opens at 10. Can I place a buy order to buy at 12 once it passes 10 ?

 

 

You would have to just set a buy stop-limit order at 12 for the stop and whatever you're willing to pay for the limit part of it, let's say 12.50.

I know if you execute a market order, you're at the mercy of the gap between the bid and ask price. My question is if you set a limit order, does that help you in any way - i'll give an example:  let's say i want to buy xyz and the last trade is at 10.  If I do a market order, I might pay 10.09 per share compared to my bid of 10. Do limit orders help me get past this gap like for instance if i set a limit on that at 11 per share?