Not new

After-hours trading has existed for sometime but mainly wealthy investors and institutional traders used the system.

ECN

Electronic communication networks (ECNs) make after-hours trading available by connecting individual traders and participating brokerages. ECNs eliminate the third party required to make a transaction. ECNs also give investors real-time information unlike the NYSE that only displays the best bid and ask price.

Market makers

Market makers help facilitate after-hours trading. Market makers represent broker/dealer firms that risk holding specific securities in order to give investors the opportunity to buy and sell the securities at anytime.

Convenience

Investors send buy or sell orders 24 hours a day and as long as a market maker for the stock exists the ECN matches the opposing orders (buy with sell) to fill the trade. ECNs match orders during extended hours that vary by ECN but typically from 7:30 a.m. to 9:15 a.m. (pre-market) and from 4:15 p.m to 8 p.m. (after-market). Typically brokerages handle only limit orders during these hours.

Risks

The Securities and Exchange Commission warns of the risks associated with after-hours trading. The market becomes subject to more volatility and less liquidity during after-hours sessions. The SEC advises investors to read its publication, "After-Hours Trading: Understanding the Risk."

Source:

Investopedia.com: Electronic Trading - Electronic Communications Networks

SEC.gov: Answers - After-Hours Trading