There are about a thousand of them listed on the U.S. exchanges.

They track everything from the S&P 500… to gold… to Treasury bonds… and much more.

#-ad_banner-#They are basically nothing more than portfolios of stocks, bonds or commodities that trade on the major exchanges as a single security. But underneath a placid exterior, one of America’s fastest-growing asset classes reached a key milestone just a few short years ago: Total assets invested in U.S. exchange-traded funds (ETFs) surpassed $1 trillion for the first time.

That represents the culmination of a remarkable episode of growth. The first U.S.-traded ETF was launched on January 29, 1993 — so it took fewer than 19 years for the ETF industry to crack the $1 trillion barrier.

To put that in perspective, it took the mutual fund industry (first launched in 1924) 66 years to surpass $1 trillion in assets.

Assets invested in ETFs grew at a 31% annualized pace from 2000 to 2011 compared to just 6% annual growth for mutual funds. And alongside the growth of ETFs is the growth in closed-end funds (CEFs).

The differences between CEFs and ETFs are small — both allow you to buy into a basket of securities with one simple transaction. And you can buy them throughout the day, just like a stock.

Yet many investors don’t realize just how powerful a tool ETFs and CEFs can be for income investors.

Let me give you an example. If you’re a subscriber to my premium newsletter, High-Yield International, then you’ve heard me talk before about the abundance of high-yielding stocks around the world. At last count, we found 204 companies based outside the U.S. paying more than 12%.

And there are hundreds of foreign stocks listed on the major U.S. exchanges. But there are tens of thousands of foreign stocks that don’t trade in the U.S. Buying these stocks can be difficult for the average investor.

ETFs and CEFs break down all these barriers, allowing investors to buy broadly diversified baskets of dividend-paying foreign stocks without paying higher commissions to their broker or dealing with foreign currencies.

In short, they can make buying a portfolio of high-yielding stocks from say, Brazil, as simple as buying a share of Wal-Mart.

And thanks to these funds, investors can capture some pretty significant returns and yields that would otherwise be untouchable.

In my High-Yield International portfolio, we held shares of the AllianceBernstein Global High Income Fund (NYSE: AWF) from March 2009 through April 2014. AWF invests in bonds from around the world — including foreign-government bonds from emerging markets and U.S. corporate high-yield bonds.

Normally it would be next to impossible for average investors to buy the securities AWF holds. But with the fund, you can access them all in one simple transaction.

In High-Yield International, we locked in a 17.1% yield when we purchased AWF. And during the time we held the fund, it delivered a total return of more than 210.9%, including dividends, to anyone who followed our lead.

Today, even after the rebound, the shares still carry a 9% dividend yield. (What you actually earn could turn out to be significantly higher than that, as the fund usually pays shareholders with a special dividend each December.)

Despite these positives, these funds aren’t perfect. Investors can often earn higher returns by picking individual stocks and there are management fees that cut into returns.

But by allowing serious income investors to access some of the highest-yielding securities on the planet as simply as a share of General Electric, Wal-Mart or Apple, the extra costs are usually worth it.

Note: As I said, there are enormous numbers of high-yielding investments outside U.S. borders. I’ve found more than 200 of these companies paying 12%-plus yields… and hundreds more paying above 6%. But most U.S. investors have no idea that these securities even exist.

Meanwhile, many of the world’s wealthiest investors — including Warren Buffett — have been quietly cashing in on them for decades. Check out this free presentation to get more details about these 12%-plus yields.