When you buy an “equity security,” like the common stock of a corporation, you become an owner of the company. The value of your interest fluctuates with the fortunes of the company; if the company does well the value of your interest goes up, while if it does poorly the value goes down, possibly all the way to zero. As an owner, you generally have the right to share in any profit distributions made by the company, and you also share in the appreciation in the value of the company. When a company dissolves, the owners of the equity securities are paid last, after all the creditors.

In some cases, a company will offer a “preferred equity security,” like the preferred stock of a corporation. Typically, the holders of the preferred equity security have a right to receive distributions before the holders of the regular equity securities. For example, the holders of a preferred stock might have the right to receive a 4% dividend before dividends are paid to the holders of common stock. But preferred equity is still equity. The holders of preferred equity are paid after creditors.

These securities start out as debt securities but can be changed – converted – into equity. For example, a company might issue a debt security, which compensates the holder with a defined interest rate, and that can be converted by the holder into common stock at some specified time. The conversion is triggered upon the occurrence of a specified event such as an additional fundraising round, and holders of the convertible note generally enjoy a discount  with respect to the company valuation during the liquidity event.

“SAFE” stands for “simple agreement for future equity.” SAFEs were introduced by Y Combinator in 2013 as a replacement for convertible notes. There are several kinds of SAFEs, but the typical SAFE converts into an equity security – either common stock or preferred stock – when the Issuer raises more money in the next equity fundraise. If the Company never raises more money in the future you typically have to wait until the company is sold or dissolved to get your money. The SAFE subscription agreement will define the conversion event, and sometimes a valuation cap and/or discount.

A regular promissory note requires the Issuer make specified payments of interest and principal at specified times. In contrast, a revenue-sharing note requires the Issuer to pay a specified percentage of its revenue. For example, a revenue-sharing note might require the Issuer to pay investors 5% of its revenue for four years. Typically, a revenue-sharing note will also state a maximum that investors are entitled to receive (e.g., double their investment) and a due date for repayment of the original investment.

Specifically, promissory notes. The promissory notes will require the Issuer to pay your money back, plus interest at a specified rate, over a specified time period. Owning a promissory note does not make you an owner of the company. Instead, you are a creditor. As long as the company has enough money to repay your loan, plus any interest you’ve been promised, the value of your security stays the same; the fluctuations of the fortunes of the company don’t affect you, unless the fortunes go way down. On the other hand, you don’t share in the appreciation if things go well. If the company increases in value 100-fold, you just have the right to get your money back, plus interest.