if it is a big company then the shares will be listed on a stock exchange so that the shares can be bought and sold. The most well known stock exchange is the London stock exchange (LSE) the top one hundred shares of which form the FOOTSIE 100. Listing places certain, expensive and administrative burdens on a company so only large ones tend to go for it. The plus side is that it provides a ready and highly efficient market place for your shares. So our man turns his bike shop into a 200 shop chain making about 5 million per year. he decides to go public (list his shares on a stock exchange) and splits the company up into 60 million shares and offers them for sale. The market will decide if the price being offered represnts good value. If it does then all of the shares will be snapped up. The reason for doing this are mainly to raise massive cash for expansion or to take a big cash reward for building such a large and successful company, usually a mix of both.
Once they are listed then the shares are on their own. Goods news (like a total traffic ban) will lead to an increased demand for those shares and a subsequent rise in their price. Bad news (cycling causes cancer!) shares will fall.
Often the actual profit or loss of the company will not be too relevant, shares tend to be bought and sold on future propects rather than current trading conditions.
Check out the MOTLEY FOOL website for a nice gentle and funny run through of all things financial
www.fool.co.uk
hope this helps
andy