There are three main areas of influence that move a stock's price up or down over a period of time and a fourth that manifests these changes in daily trading. If you understand these influences, it will help you decide whether the price movement is a buy, sell or sit tight signal.
Let's first get the fourth influence on a stock's price: supply and demand. Supply and demand is how traders perceive the other influences on the market. If more people are buying than selling, the price will rise. If more people are selling than buying, the price will fall.
While some major news event or change in economic indicators may give a sudden push to prices (or drag), over the long term the following three influences will help determine whether investors are buying or selling the stock.
Clearly, the most direct influence on a stock's price is a change in the economic fundamentals of the business.
If revenues and profits are on a steep upward trend with no indication of leveling off, you can expect to see the stock price rise as investors bid up this attractive company.
On the other hand, if the profit picture is flat or, worse, declining with no change in sight, look for investors to abandon the stock and the price to fall.
These are simple examples of changes in fundamentals. Other, more complex and subtle changes can occur that may not dramatically affect the stock price immediately (increased debt, a poor acquisition and so on can also trigger price changes).
The point is that changes in the underlying business have a direct impact on the stock's price. Smart investors spot the subtle changes before they become price-movers and take the appropriate action.
Changes in the stock's sector can have positive or negative affects on price too. Some sectors or industries are cyclical in nature and you should know that would affect price.
However, when whole sectors catch of fire (think dot.com stocks) or burn up (think dot.com stocks, again), even those companies that have solid fundamentals are pulled along with the rest of the sector.
You may hold a stock that is a victim of "guilt by association" when an industry falls out of favor. Likewise, stocks can see prices artificially inflated if they find themselves in the right industry at the right time. This is another good reason to work on keeping a well-diversified portfolio that will not tank if one sector does.
The market goes up and the market goes down. That's about all you can say with certainty concerning the stock market.
As the market moves up and down, your stock may move with or against it. Most large-cap stocks will follow the market to some degree, but smaller companies may not get the same push every time.
In general, a strong market move either up or down will carry more stocks with it than not, so your stock may be up or down for no other reason than the market was up or down.
How do you use this information? A change in fundamentals may be an opportunity to buy more shares of a growing company or it may signal the time to sell if the changes are for the worse.
A change in the sector is usually temporary so most long-term investors will ride out dips due to these factors. However, if something drastically changes in the stock's industry due to regulation or a new technology, for example, you may want to reevaluate your position. Is the company capable of adapting or do you own a dinosaur?
Market swings that move your stock's price can be opportunities to buy additional shares (assuming all the company's fundamentals still checkout). If the rising market pushes up your stock's price, it may be time to take a profit on part of your holdings and wait for the price to come back down to earth to reinvest.
The stock market is extremely complex and the above is only a very high-level view of what happens on a daily basis. However, keep your eye on these and you will be better prepared for opportunities.