Level I data, also known as top-of-book data, is an unfiltered tick-by-tick feed of the last traded price, plus the best bid and best ask price, in any given futures market.
Level II data, also known as full market depth or order book, is the same feed as Level I, PLUS data on the number of contracts that are resting at prices above and below the last traded price.
Level II data is also required to view certain indicators in Tradovate's Order Flow+ package.
Please note that Level II data is not inherently better or faster than Level I data; it just contains more information that could be used by some traders.