What is a stock split and what effect does it have on the company's stockholders equity?

When Yahoo! Inc reported in early 2004 that its first quarter earnings had doubled from the previous year, its stock price jumped 10 percent to $53 per share. At the same time, the company announced a 2-for-1 stock split. What is a stock split and what effect does it have on the company's  stockholders equity? What effect will it likely have on the market value of the company's stock? In light of your answers, do you think the stock split is positive for  the company and its stockholders? (provide any citations) Thanks.