What's an Exchange-Traded Fund (ETF)?
Wikipedia has a fairly detailed explanation of ETFs.  http://en.wikipedia.org/wiki/Exchange-traded_fund
Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
What's an Exchange-Traded Fund (ETF)?

### Response:
Wikipedia has a fairly detailed explanation of ETFs.  http://en.wikipedia.org/wiki/Exchange-traded_fund