Complex strategies such as large delta-neutral futures and options combination positions, or unusually large lot futures trades are executed as block trades and are traded directly between two participants in a pre-arranged trade permitted by the exchange. These can be seen on time and sales, and last trade data, but they do not go through the market - so if you have an order resting in the book, you will not get a fill.
Block trades are generally deemed by most traders to be grossly unfair since a large number of these trades could be put through the market place and add much needed liquidity and additional volatility. As far as the exchanges are concerned, they dont really give a monkeys since they get their fee's per lot of the block trade for doing nothing. Most block trades are executed off the book over the telephone by big banks and generally reek of croneyism.