Stocks have two types of valuations.  One is a value created using some type of cash flow, sales or fundamental earnings analysis. The other value is dictated by how much an investor is willing to pay for a particular share of stock and by how much other investors are willing to sell a stock for (in other words, by supply and demand). Both of these values change over time as investors change the way they analyze stocks and as they become more or less confident in the future of stocks.  Let me discuss both types of valuations.

First, the fundamental valuation.  This is the valuation that people use to justify stock prices.  The most common example of this type of valuation methodology is P/E ratio, which stands for Price to Earnings Ratio.  This form of valuation is based on historic ratios and statistics and aims to assign value to a stock based on measurable attributes.  This form of valuation is typically what drives long-term stock prices.

The other way stocks are valued is based on supply and demand.  The more people that want to buy the stock, the higher its price will be.  And conversely, the more people that want to sell the stock, the lower the price will be.  This form of valuation is very hard to understand or predict, and is often drives the short-term stock market trends.