Every day, we buy, store and consume commodities. When we go to the gas station or grocery store, we are directly participating in the commodities market; we're just not investing in it.

It used to be fairly difficult for retail investors to get exposure to commodities. But as with so many other sectors, exchange-traded funds (ETFs) have changed all that.

Traders have always known that investing in the commodities market can be lucrative. Before ETFs, traders were limited to buying and selling futures contracts on the underlying commodities or buying and selling stocks of companies that produce commodities. Each strategy had its pitfalls.

Thank goodness for ETFs. They make the investing process a lot simpler, but they still need to be inspected closely so you know exactly what you're investing in. With that in mind, let's have a look at some of the most popular commodity ETFs.

SPDR Gold shares (NYSE: GLD)

GLD is the second-largest ETF in the world with almost $50 billion in assets under management, trailing only the SPDR S&P 500 ETF (NYSE: SPY). GLD owns more gold than many of the world's central banks, including China's, making this ETF the sixth-largest owner of physical gold in the world.

GLD has proven popular with investors looking for gold exposure because each share of GLD is backed by physical gold bullion. So if you want to invest in physical gold but have nowhere to store it, GLD is a great alternative. There are several other ETFs that are also backed by physical gold, but GLD is by far the largest and boasts the greatest liquidity.

iShares Silver Trust (NYSE: SLV)

SLV is to silver as GLD is to gold: an ETF backed by physical holdings of a precious metal. Silver is often viewed as gold's less popular cousin, but the case for investing in silver is strong, particularly when gold prices are high. Because there's more of it, silver prices will always be lower than gold prices. But silver, unlike gold, has industrial applications which can buffer prices from the volatility often seen in the gold market.

With about $5.5 billion in assets under management, strong daily trading volume and an expense ratio of just 0.5%, SLV is the best way for investors to tap into silver.

U.S. Oil Fund & U.S. Natural Gas Fund (NYSE: USO, UNG)

If commodity ETFs get a bum rap, chances are it's because of USO and UNG. These ETFs have always been controversial because they don't invest in oil or natural gas, but instead invest in the futures contracts for oil or natural gas. As the futures contracts expire, UNG and USO are forced to roll-over their holdings by purchasing new contracts every month. If the markets are in contango (meaning the future price of the commodity is higher than today's price), then these ETFs are essentially selling low and buying high.

This has been a huge problem for USO and UNG because the oil and natural gas markets have been in contango for most of the ETFs' lives. If the markets for these commodities return to backwardation (future prices are lower than today's prices) then UNG and USO will perform better.

PowerShares DB Agriculture Fund (NYSE: DBA)

If you think the volatility of investing in energy and metals is too much to handle, than you'll probably want to stay away from ETFs that invest in agriculture commodities, known as 'softs.' These commodities offer the potential for rapid capital appreciation, making DBA an ETF to keep on your radar.

DBA's issuer has said it is looking to add more equities to the ETF's holdings. That could help reduce expenses and volatility in the future, making the ETF a bit more palatable for conservative investors.

ETF Securities Physical Palladium & Platinum Shares (HYSE: PALL, PPLT)

ETF Securities offers a broad swath of commodities-based products on Asian, American and European exchanges. In addition to gold and silver ETFs, it offers the only physical palladium- and platimum-backed ETFs available to U.S. investors.

PALL and PPLT invest in metals that have a variety of industrial uses, including the production of catalytic converters. Platinum and palladium prices are among the most volatile in the metals complex, making these ETFs appropriate for investors willing to accept a high level of risk. Both ETFs have expense ratios of 0.6%.