A stock market bubble occurs when asset prices rise far beyond their intrinsic value, driven by excessive speculation and investor euphoria rather than fundamentals. Classic signs include rapid price surges, high trading volumes, and widespread media hype. For example, the dot-com bubble of the late 1990s saw tech stocks soar despite many companies having no profits. To protect yourself, focus on valuation metrics like P/E ratios, diversify your portfolio, and avoid chasing hype. Bubbles often burst when sentiment shifts, so staying disciplined is key.