What are "Dark Pools" as discussed in the book, Flash Boys?

Dark Pools are privately traded stock exchanges, or alternative trading systems, often offered by banks or large corporations. Trading in a dark pool can be riskier because information is only made available to the host. For this reason, dark pools are favorites of high-frequency traders. Investors place their money in a dark pool in the hopes of alerting others that a big-seller has entered the market, only to have it ravaged by HFT.