Mom-and-pop investors have crossed an important threshold in how they're putting their money to work.
Over the past 12 months, the group, through registered investment advisors, brokerages and other retail sources, has put more money into exchange-traded funds than mutual funds, according to Broadridge Financial Solutions, an investor communications firm. That has not happened before.
Read MoreWhat's an ETF?
While there's still a huge mismatch in terms of total money under management between the two industries—$2.1 trillion for ETFs, $13.5 trillion in long-term mutual funds, according to the Investment Company Institute—the trend is worth noting for several reasons.
Conventional wisdom had long held that ETFs were more the playground of traders who use the index-tracking funds in part as hedging tools because they are compiled like mutual funds but can be traded like stocks. Mutual funds, conversely, were seen as long-term investments because they are less liquid.