The accredited-investor gate is the single most important thing to understand before looking at any specific alternative, which is why it is worth stating once here rather than repeating on each page. An accredited investor is a person or entity that meets the SEC's Rule 501 thresholds, most commonly a net worth over $1 million excluding the primary home, or income over $200,000 (single) or $300,000 (joint) in each of the last two years. Certain professional licenses also qualify a person regardless of wealth. Accreditation means you are permitted to take the risk; it is not a regulator's judgment that a deal is sound. Some offerings go further and require a qualified purchaser, broadly an individual with $5 million in investments, which is a much smaller pool than accredited investors.
The fee structure is the second shared feature, and it compounds. The private fund standard is "2 and 20": a 2 percent annual management fee charged on committed or invested capital, plus carried interest of 20 percent of profits, usually only above a stated hurdle rate and subject to a high-water mark. Those fees are an order of magnitude above a broad index fund's expense ratio, so an alternative has to clear a high bar just to match a cheap public portfolio after costs.
The third feature, valuation opacity, is subtler and cuts against the marketing case. A private fund reports the value of its holdings by appraisal, typically each quarter, rather than by a market price set every second. That appraisal smoothing makes reported returns look steadier and less correlated with public markets than the underlying economics actually are. Some of the diversification benefit an alternative appears to offer is a measurement artifact of infrequent, manager-influenced pricing, not a real reduction in risk. A short menu of the main children of this category, each covered on its own page:
Private equity and venture capital: ownership stakes in private companies, mature and early-stage respectively.
Angel investing: an individual, rather than a fund, backing the earliest startups.
Hedge funds: pooled private funds using strategies mutual funds cannot, such as short selling, leverage, and derivatives.
Private credit: non-bank lending to companies through funds.
Commodities: raw materials such as energy, metals, and agriculture.
Non-traded REITs and structured products: securities sold through brokers with the liquidity and cost hazards of private products.
Collectibles: physical objects held for price appreciation, taxed under their own higher capital-gains ceiling.