Stock prices are a direct result of supply and demand. All the other influences like debt, balance sheets, earnings and so on affect the desirability of owning (or selling) a stock. This article details why supply and demand create changes in stock prices, and what a drop in price means in terms of supply and demand.
Stock market prices are affected by business fundamentals, company and world events, human psychology, and much more. Read this article for descriptions of a few of the main factors than affect stock market prices.