Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you'd like to add some mid-cap stocks to your portfolio because they're big enough to have proven themselves to some degree and yet small enough to still have lots of room to grow, the Vanguard Mid-Cap Growth ETF (NYSE: VOT) could save you a lot of trouble. Instead of trying to figure out which companies will perform best, you can use this ETF to invest in lots of them simultaneously.

The basics
ETFs often sport lower expense ratios than their mutual fund cousins. The Vanguard ETF's expense ratio -- its annual fee -- is a very low 0.10%. (Vanguard is known for low fees.)

This ETF has performed reasonably, inching ahead of the S&P 500 over the past five years and beating it over the past three. As with most investments, of course, we can't expect outstanding performances in every quarter or year. Investors with conviction need to wait for their holdings to deliver.

What's in it?
More than a handful of growing mid caps had strong performances over the past year. Edwards Lifesciences (NYSE: EW), a leading manufacturer of cardiovascular devices such as its Sapien heart valve, surged 36% over the past year. Its field promises lots of customers as our growing population ages, but Edwards is not without competition. Recent health care reforms include a new tax on medical devices, but many don't expect this to hurt the industry too much. Some wonder whether the company will end up being bought out, but that's just speculation at this point.

Vertex Pharmaceuticals (Nasdaq: VRTX) gained 15%, with some viewing it as a potential monster stock of the future because of its promising cystic fibrosis treatment, Kalydeco. Its sales are growing briskly, and approval in Europe is expected soon. Vertex is also working on hepatitis C treatments, though it has strong competition on that front -- one of two such treatments recently fared well in phase 1 trials. The company got a bit of bad news, though, when some young rats treated with Kalydeco developed cataracts. It's not clear if this will happen to humans, but the FDA has issued an alert and has called for further investigation. Meanwhile, Vertex compares favorably valuation-wise with several peers.

Other companies didn't do as well last year, but could see their fortunes change in the coming years. Health care information services specialist Cerner (Nasdaq: CERN) gained just 2%, but it's no slouch, having averaged 19% growth annually over the past two decades. An insider recently bought $1 million worth of shares, which is a sign of confidence. President Obama's health care reforms include a mandate to move much of medical record-keeping onto electronic formats, which also bodes well for Cerner.

Chipotle Mexican Grill (NYSE: CMG), meanwhile, shed 1%, after averaging 22% annual growth over the past five years. The stock has fallen in recent months after the company reported a slowdown in customer traffic. On the plus side, it's growing briskly and sports a very healthy balance sheet. But its growth rate can't remain steep forever, though some see the company as having secret weapons to call into use, such as extended hours.

The big picture
A well-chosen ETF can grant you instant diversification across any industry or group of companies -- and make investing in and profiting from it that much easier.

If you'd like to invest in some companies set to dominate the world, check out our special free report, which will introduce you to three, two of which compete with Chipotle.