The bond market (also called the debt or fixed income market) is where governments and companies borrow money by issuing bonds to investors. It is actually larger than the stock market — the global bond market exceeds $130 trillion vs. $110 trillion for equities. When people talk about "the markets," they often forget that the bond market is the bigger, quieter sibling.
Key segments: government bonds (Treasuries, G-Secs) — safest; corporate bonds — higher yield, more risk; municipal bonds ("munis") — tax-exempt in the US; and mortgage-backed securities — pools of home loans. The US Treasury market alone is $25+ trillion and sets benchmark interest rates for the entire world.
The bond market is often called the "smart money" market — bond investors tend to be more sophisticated than stock investors. When bond yields spike (prices fall), it signals economic trouble. The 10-year US Treasury yield is the most watched number in finance — it influences mortgage rates, corporate borrowing costs, and stock valuations globally.