While I'm on here explaining the wonkish details of the Robin oHood Tax, I'll answer another question that's had some of you scratching your heads today: is the Robin Hood tax is the same thing as the Tobin Tax which people have been campaigning for since the 1970?
The FTT is certainly pretty close to the Tobin tax - but like financial markets themselves, the idea has grown in the 40 years since James Tobin first proposed it. Tobin's idea was just to tax currency trades. At that time, all the other things that financial markets trade now, like derivatives and bonds, swaps and futures, were just a gleam in Goldman Sachs' eye. But now, of course, they dominate the market. So Tobin's original idea has been extended to those products too, to reflect the realities of how markets work today.
As the market has got more complicated, so the ways of taxing it have too. The main difference is that while Tobin proposed just one tax rate for everything traded on the market, the Robin Hood Tax proposal is that different bits of the financial market might be taxed at different rates, to reflect the profitability of trades in those markets. If a tax were introduced, the markets would have to be monitored carefully until the 'right' tax rate for each market was found - one which raises money without killing off the market.