The limit you place to protect you trade, if you buy/sell some currency, the trade can be protected by placing stop loss at particular price. The trade automatically stops when it hits the stop loss

For a short trade stop Loss Stays below the order and for the long trade the stop loss stays above the order you placed. Stop loss is not fixed at one point. We can adjust the stop loss by moving it.

The profit limit for your trade position, if any of the position is running or even while placing a pending order you can use take Profit , Simply take profit means where your trade should close if it in running in profits

For a long position take profit stays above the order and stays below for short position. For Pending Orders Take profit and Stop loss are compulsory along with size and position price.

Trailing stop means the stop loss which stays always behind the order you placed, so it is below/above the order, trader should give it a PIP value, that is how many pips  it should trail/stay behind. For example if you give Trail value as 20 pips/200 piplet's it always stay 20 pips below the price moves behind the price as long as price hits the trails stop. 

If the price starts moving in the opposite direction, trail stop stays in last position only. So, trail stop is one way road it moves as long as price moves when price reverses it stay in the last blocked position and price hits it trade closes.

Trail stop is always good as it saves the profit but sometimes market is in correction mood it moves back to opposite trend for some time and starts it journey into the direction of trend, in this kind of scenario price hits the trailing stop and trader may lose big profit.