A Tobin tax is a tax on financial transaction's Tobin's original proposal was to tax foreign exchange transactions, to both discourage short term specualtion and to raise revenue in a fairly painless way.
More recent proposals have expanded the idea to taxes on a broader range of financial transactions, and proposed to specifically target banks. This type of tax is sometimes called a Robin Hood tax to distinguish it from the original Tobin tax.
Objections to the Tobin tax
The Tobin Tax has obvious attractions, but it has a number of disadvantages. The greatest of these is that an effective Tobin Tax needs to be imposed globally: otherwise traders can evade it by shifting their trading to another country.