economics of trading

[playvibes]

cud some1 explain me the science/economics behind the rise n fall of share prices. rite now all i know is that the demand in the market plays some role in the stock prices.
Is it like this :
a) if more people buy the company's shares , its stock rates go up
b) if more people sell shares of da co. then prices fall
Well what r the other factors responsible for rise n fall .... ????