A LIT market, or light pool market, refers to ECN stock exchanges where the order book is made public for all who subscribe. A LIT or light pool market will allow traders to see the amount of liquidity that is posted on the bid and offer of the order book for a security. A trader will use the information that they see on the LIT markets as an indication of the stocks likely near term direction.
A majority of volume still transacts over the light pool markets. According to a recent Wall Street Journal Article, about 70% of volume is transacted in the LIT markets.
It is easy to spot a large buyer or seller when they post orders to LIT markets, because the large order will show up on the book for all to see. Because traders will run the price away from the large order, large buyers and sellers have become very clever about transacting on the light pools but they still are usually forced to show themselves to a large degree.
Examples of LIT ECNs are BATS, BATS BYX, ARCA, EDGX, EDGA, NASDAQ, and NASDAQ Boston. Each light pool has a different pricing scheme, and some will pay rebates for adding liquidity while others will pay rebates for removing liquidity. A trader must understand pricing intricacies of each market in order to maximize his own cost efficiency, as well as to understand which markets will be the most liquid.
A trader with a longer term holding strategy who only places limit orders for profit will most likely always sit on the market that pays him the biggest rebate, and in some cases a trader can even make money from his transactions alone. On the other hand if the trader seeks the most aggressive liquidity he will increase his trading costs, but will improve his ability to get executed at the price he wants.
Eventually when you have been trading for a consistent period of time choosing the right LIT ECN for your needs becomes second nature.