Each market maker supports a select number of companies. At all times, each market maker posts a bid and an ask. That is called the spread.
Along with this spread, they post a quantity that will read like this: 200 x 400
That means they will BUY 200 shares at the bid price, and they will SELL 400 shares at the ask price. That shows you that they are trying to unload shares. It also means the price is probably dropping.
If it were this: 400 x 200
It would mean that they will BUY 400 shares at the bid price and they will sell 200 at the ask price. That shows that they are trying to load up. They are willing to buy more shares than they are willing to sell at that spread. That probably means the share price is increasing.
In level 2, you will see these 200's and the 400's as trades. They are trades, but they are part of a larger order. After each 200 or 400, the MM can change the bid, the offer, and the quantities. That's why, when you buy a bunch at market, the price slowly increases. You pay more for each tranch, because the MM computers can see that it is a market order, and it is a large order, so they ramp up their ask as they go through each quantity. When you finally see your order, you can look at your brokerage and see how many you got at each price. I once had a million share order, and one tranch was for only 1 share. That's because 999,999 shares were traded through other tranches, and that one share was left to complete the order. That would always be the last tranch - it is what is left at the end.
Look up a stock on Yahoo. Look at the bid/ask prices. They will look like this:
Bid 4.9200 x 42300
Ask 4.9300 x 29200
Day's Range 4.8900 - 4.9500
That means they will buy up to 42300 shares at 4.92, and they will sell up to 29200 shares at 4.93. They will make a penny per share traded. It also shows that they want to buy more shares than they want to sell.
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