The European Central Bank (or ECB) is the central bank for the eurozone. Similar to the Federal Reserve of the Bank of England, the ECB sets monetary policy for many countries in Europe. The fact that it serves several countries (ie, every country who uses the Euro) differentiates it from other central banking institutions. Though a relatively young institution (founded in 1998), the decisions made by the ECB have enormous impact on traders and forex markets.
We don't know everything about the markets. We're just devoted to learners. Taken from those smarter than ourselves, here's how we define ECB.