Top of Book data, or Level I data, displays the current best Bid and Ask price for a market, as well as the volume of working orders on the Bid and Ask . Below is an example of how a DOM would look with Level I data:

Level I data is often used by traders who only trade with Charts, or traders who do not have a need to pay higher data fees to get Level II data. 

Depth of Market, or Level II Data, displays the current best Bid and Ask for a contract, as well as additional levels of working orders for that market. The number of levels displayed can vary by data feed.  Rithmic data, for example, provides unlimited depth on all contracts unless the depth is restricted by the exchange. Most data providers will provide 10 levels of Bid prices, and 10 levels of Ask prices with Depth of Market data. An example of Rithmic's DOM with Depth of Market data can be seen below: