Along with the birth of a number of exchange traded fund (ETF) indexing strategies, there are numerous product structures to choose from. Picking one ETF product structure over another will affect how dividends are paid, the margin of tracking error and taxes. Ron DeLegge for ETFGuide looks at the key legal structures and provides a brief summary:

• Open-end index fund The majority of ETFs follow this structure, as it allows the most flexibility. Dividends on this type of fund are reinvested and paid to shareholders. Families include iShares, SPDRs, PowerShares, Vanguard and WisdomTree.
• Unit Investment Trusts (UITs) These are the oldest and most famous ETFs and they include BLDRs, Dow Diamonds (DIA), SPDRs, and PowerShares QQQ (QQQQ). Dividends aren’t reinvested, simply paid out quarterly or annually. They carry expiration dates that are rolled or extended.
• Grantor Trusts Original securities are not rebalanced and remain fixed. Dividends are distributed directly to shareholders and allows investors to retain voting rights on underlying securities within the trust. streetTracks Gold Shares (GLD), iShares Silver Trust (SLV), HOLDRs, and Rydex CurrencyShares follow this format.
• Exchange traded notes (ETNs) These are debt instruments that pay a return linked to the performance of a single security or index. ETNs are treated and are currently taxed like prepaid contracts.
• Partnerships Some  commodities are treated like a master limited partnership (MLP). Unit holders are to report their share of MLPs income, gains, losses and deductions on federal tax returns even if there are no distributions. Form K-1 is used for tax reporting.

Tags: Currency ETFs, Dividend ETFs, ETNs, GLD, Silver