In simple words the answer could be "Demand & Supply". The share will rise if there's more demand for shares of company companies and less people willing to buy them. The share will fall if there is an inordinate amount of supply, but no one is willing to buy shares at the current price.
The big question now is: How does additional demand come into the market? There are many factors involved in this, including the company's financial results, overall economy performance, sector performance and government rules & regulation. These factors also affect the future of the company, future products & services, management changes, stock exchange frauds, and company management.
All of these factors and many others can affect the demand and supply of company stock and eventually cause it to move up and down.
It is difficult to predict stock movements and requires a lot of research and expertise.