Editor's note: As part of our partnership with PBS's Nightly Business Report, TheStreet's Alix Steel joined NBR to explain what you should know before investing in gold or silver ETFs.(Watch video here.)

NEW YORK ( TheStreet ) -- Silver and gold prices are hitting record highs, making physically backed exchange-traded funds very tempting investments. Before you buy, here's what you need to know.

There are five physically backed ETFs traded in the U.S.: The biggest, SPDR Gold Shares ( GLD); the cheapest, iShares Comex Gold Trust ( IAU); the newest ETFS Physical Gold Shares ( SGOL) and the two silvers, iShares Silver Trust ( SLV) and ETFS Physical Silver Shares ( SIVR).

When you buy gold and silver physically backed ETFs, you do not own the physical metal, you own a paper representation. With respect to the gold ETFs, for every share you buy, you "own" one tenth of an ounce of gold; for silver, it's one ounce.

The actual metal is stored by a custodian, usually one of the large banks like JPMorgan ( JPM) or HSBC. The share-to-metal correlation erodes the longer you hold the shares. The fund must sell gold, for example, periodically to pay for expenses which decreases the amount of gold allocated to each share.
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If investor demand outpaces available shares then the issuer/trustee must buy more physical metal to convert it into stock. Conversely, when investors sell, if there are no buyers, then the metal is redeemed, the trustee must then sell the metal equivalent. Precious metal ETFs are not owned for leverage, but simply as a vehicle to track the spot price.

Investors are not typically encouraged to redeem their shares for the metal although it is possible. With respect to the SGOL, for example, an investor would have to redeem in whole lots of 50,000 shares (5,000 ounces, or $6.65 million) but only through an authorized participant. This is more feasible for high net worth individuals and funds but not really for retail investors.

Because you own shares and not the physical metal, precious metal ETFs may be sold short, so two people can own the same "gold" -- the original owner and the investor who is borrowing the shares. Although baskets of shares are allocated to specific gold bars, which can be found in the ETF's prospectus, an investor must share ownership.

Owning a precious metal ETF has different fees than owning and storing the physical metal. Expense ratios can range from 0.25% to 0.50%, while storage fees at GoldMoney.com, according to founder James Turk, for example, cost 0.15% to 0.18%. In addition, the cost of buying the physical metal at GoldMoney.com can cost anywhere from 0.98% to 2.49% while buying the ETF comes with a traditional broker's fee.

Profits made on investments in physically backed ETFs are taxed like collectables, at a maximum of 28%, if the investment is held for more than a year. If an investor sells before the year is up, he is taxed at his regular income rate. Basically an investor gets taxed as if he owned bullion, when in reality he just owns paper.

There are also two types of gold stored in the ETFs, allocated and unallocated. Allocated gold is the bullion held by the custodian. Custodians provide a bar list of all the individual allocated bars daily and are typically audited twice a year, paid for by the sponsor, by an independent party like Inspector International.

Unallocated gold relates to authorized participants like JPMorgan or Goldman Sachs who trade gold futures. Futures contracts are often bought if the trustee needs to create new shares fast and doesn't have the time to buy and deliver the bullion. Typically allocated gold far outweighs the unallocated gold and the amounts are tallied each day by the custodian.

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