Exchange-traded funds offer a convenient way to invest in sectors or niches that interest you. If you expect the media industry to thrive over time, especially with a big presidential election season heating up, which will involve a lot of advertising, the PowerShares Dynamic Media Portfolio ETF (NYSE: PBS) 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 media ETF's expense ratio -- its annual fee -- is 0.63%, which is a bit higher than many ETFs, but also considerably lower than most stock mutual funds.

This ETF has been a mixed performer, beating the S&P 500 over the past three years but lagging it over the past five. 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?
Several of this ETF's components made strong contributions to its performance over the past year. Sinclair Broadcast Group (Nasdaq: SBGI) gained a whopping 44% over the past year, getting its biggest boosts early in the year from political ad spending and the Super Bowl, among other things. It also bought more stations and reinstated its dividend, recently yielding 4.2%. Meanwhile, HSN (Nasdaq: HSNI), parent of the Home Shopping Network, advanced 19% -- finding success with its online gaming site, HSN Arcade, as well as e-commerce ventures and its growing mobile operations.

Other companies didn't add as much to the ETF's returns last year, but could have an effect in the years to come. Advertising giant Interpublic Group (NYSE: IPG) shed about 6%, delivering relatively weak earnings numbers in a tough economic environment. The company is seeing promising growth in emerging markets, though, and collected a $132 million gain recently from selling half its stake in Facebook.

Professional networking site LinkedIn (NYSE: LNKD) doesn't yet have a full year on the books, but its shares opened around $83 in May and were recently near $63. Along with Dunkin' Donuts and Groupon, it was an underwhelming IPO this year. Not all the underwhelming IPOs are equal, though. LinkedIn's members are more likely to stick around than Groupon's, as its collection of people would be hard for a competitor to duplicate.

The big picture
Demand for media isn't going away anytime soon, though media formats do change over time, with newspapers having a rough time lately. 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.

Learn about the best dividend ETFs. And if you're looking for some great investments beyond ETFs, consider these 10 Stocks for Your Retirement Portfolio.