The Hammer is a single candle bullish pattern, which comes at the end of a downtrend. The body forms at the very top of the intraday range with a tall lower shadow and no upper shadow (or very small upper shadow). The long lower shadow is supposed to imply buying pressure at the lower levels.
The Inverted Hammer or Hanging Man looks exactly the same as the Bullish Hammer but the interpretation is different due to the context. The Hanging Man appears at the top of an uptrend and in this case, the long lower shadow reflects the bears trying to push the price down.