Short answers, pulled from the story.
A security is a tradable financial asset. The category includes debt securities such as bonds and commercial paper, equity securities such as common stocks, derivatives such as futures and options, and hybrid instruments such as convertible bonds and preference shares.
Debt securities entitle the holder to regular interest payments and repayment of principal, regardless of the issuer's financial performance. Equity securities carry no guaranteed payments; instead, equity holders receive a proportional share of profits and capital gain, along with voting rights, and rank last in a bankruptcy after all creditors are paid.
In the primary market, investors purchase securities directly from the issuer, providing capital to the company or government. In the secondary market, those securities are traded between investors, with money passing from buyer to seller rather than to the issuer.
The Depository Trust Company, or DTC, is owned by approximately thirty of the largest Wall Street firms and holds a single global certificate representing all outstanding securities of a class on behalf of those participant firms. Retail investors hold their shares through a chain of intermediaries that ultimately traces back to DTC, an arrangement called beneficial ownership or owning in street name.
Bearer securities are financial instruments that confer rights to whoever physically holds them, transferable simply by passing the document from person to person. Regulatory and fiscal authorities restrict them because they can facilitate tax evasion and circumvent regulations. In the United Kingdom, the Exchange Control Act 1947 heavily restricted bearer securities until 1953, and in Luxembourg a law of the 28th of July 2014 mandated their compulsory deposit with an authorized depositary.
Forced conversion occurs when a convertible bond is also a callable bond and the issuer exercises its right to call it. The bondholder then has about one month to convert the bond into ordinary shares; if they do not convert, the company pays the call price, which may be less than the value of the shares the holder would have received on conversion.