The price to sales ratio (PS ratio) is calculated by dividing stock price by the revenue per share. It is most useful for comparing companies within a sector or industry because "normal" values for this ratio vary from industry to industry. In general, low price to sales ratios are more appealing because they suggest that a company is undervalued.
An example illustrating why PS ratios should not be compared across industries: On June 21, 2010, Starbucks had a PS ratio of 1.12 while Yahoo! had a PS ratio of 2.56. In other words, Yahoo! shareholders were paying $2.56 for $1 of sales while Starbucks shareholders would only pay $1.12 for $1 of sales. However, at that same moment, the two companies' price to earnings ratios were virtually identical (Starbucks: 28.09 and Yahoo!: 27.78). Hence, shareholders were paying nearly the same amount for $1.00 in earnings. The PS ratios, though, are less comparable since Yahoo!'s profit margins are much higher than that of Starbucks.