Doji sticks have the same open and close price. Obviously in fluctuating currency markets, identical open and close prices may be rare, but if they are close enough then the candlestick can be said to be a Doji.
A Long-Legged Doji has long shadows protruding from it, indicating that there is considerable fluctuation on both sides of the open price, during the course of the trading period. Ultimately the period ends with the close price retracting back to the open price. It is a good signal of market indecision.
A Dragonfly Doji has only one long shadow, on the lower end of the open and close price. This indicates that all price activity during the trading period is on the lower side of the open price, but by the end of the trading period the price has moved back up to the open price. It is a good signal of a bearish trend reversal, i.e. price should now move upwards.
A Gravestone Doji is the opposite of a Dragonfly and again has one long shadow, to the high side of the open and close price. It indicates that during the price period all price activity is at the upper end, but that the price retracts back to open price by the end of the trading period. It is a good signal of a bullish trend reversal, i.e. price should now move downwards.
A 4-Price Doji is a rare event, in that for the prescribed trading period, the open, close, high and low price points are the same. Such an event is rare in currency trading and usually happens when trading is suspended.