L2 shows all the Market Makers (MMs)and their depth, their lot size and the price they are buying and selling. If you have a decent amount of money in stocks you shouldn't trade without it. L2 lineups can tell you alot about a stock and what might happen short term. I use AlphaTrade(.com)- $40+ bucks a month. If you want to make money, get it! Been watching L2 for hours each day for 2 years now so I have a fairly good understanding of what's going on with it.
Level 2 is a quotebook where you can see prices of the bids and ask of multiple MMs plus the transactions that are going through. It's usually a premium service, so you'd have to pay or qualify for it.