Stop is where you want to buy/sell at when you set it, and will be triggered when the price reaches that price that you set it at. When you are buying, lets say the price of ETH is $200, and you want to buy at $150, so you set the STOP price to be $150.
But to guarantee you buying it at $150, you can put a LIMIT, think of it as a bid, to which you buy ETH. Lets say you put a limit of $160, so a price of up to $160 will be placed to try to buy it. This is to ensure you buy it, especially when that price could be at the lowest point.
For selling it is the same. You put a STOP at when you want to sell it, lets say $250, and the LIMIT is how low you can go to outbid the other sellers, say $240. Take note that there are lots of other traders, so creating a simple open order to sell at $250 may not go through when the price reaches that price.
As for STOP order, it is basically without the LIMIT, and the order will become a market order when the price is right.
LIMITS can help you to have more chance of success, so you can try it too. 