Quote from shortie:
let's say you would like to sell 100 shares of MSFT. you see a bid on island for 27.20 and you are happy with the price, so you hit it with your sell order. You just took some liquidity from the market because now there are less bids hanging in there. On the other hand, if you were not happy with the price you might have put a limit order to sell your shares @27.50. In this case you added the liquidity because you added an ask to the market.
So, adding a bid/ask that is not immediately hit is adding liquidity. hitting a bid/ask that is already in the market is taking the liquidity.
I always thought the exact opposite of this was the case, i.e. Selling @ Bid or Buying @ Ask is providing liquidity.