As the most comprehensive, sophisticated capital market in the world, U.S market has provided numerous investment options.
To track the U.S economy: S&P 500 is one of the best proxies because it tracks the biggest 500 U.S company in terms of market value and it is supposed to be the strongest and it has the blend of growth and value in different industries. At the time of crisis, holding S&P 500 is safer than most of investment assuming the sizes of those big company allow them to navigate through the difficult time. There is a broader index: total market index that includes big cap, middle cap and small cap if you want to capture all the aspect of the economy(e.g. ITOT, Fidelity total market fund), but the performance is very close to S&P 500 itself due to those big companies taking more than 90% of the total economy. The representative ETFs for S&P500 are IVV, VOO, SPY (SPY being the most popular one)
跟踪美国经济， S&P500是最好的代理之一， 因为它代表美国最大500个公司，最强而且有各个工业，成长，价值各种组合。在经济危机的时候，手持S&P500是最比较安全的方式，因为这些大公司一般可以熬过困难时期而存活下来。还有一些更广阔的市场指数, 包括大，中等，小公司， 这更加全面的追踪美国经济（举例：ITOT ETF，Fidelity total market fund），但是由于美国大公司占了整个市场90%， 他们的表现非常相似。追踪市场的ETF有IVV， VOO， SPY（SPY是最流通一个）
To capture the bond market, there are some varieties as well, bond issued by Federal, local government and enterprises. There are some longer term treasury bill but 20-year treasury bill (TLT) only yields 1.5% now-days making bond market very unattractive. Muni bonds are attractive to people who are in high tax brackets which allow the return to be exempted from federal and local tax if the bond is issued by the local government. Bond issues by company has different ratings: AAA rating is safer with lower return; Junk bond is not safe but return is high.
Traditionally, people like to hold balance of stock and bond for better risk-adjusted return. Given that it is low rate environment, to achieve better return, there is a strategy called risk-parity trade, which adds leverage to bond, thus the return is similar to stock even with a combination of bond and stock.
一般来说，人们喜欢持有股票和债券的组合，波动性较小。但是最近是低利息时间，为了更好的收益， 有一种投资策略是risk-parity, 在债券方面增加杠杠，收益就可以和股票差不多。