How a stock price is determined
A stock price is determined by what was last paid for it. During market hours (usually weekdays from 9:30AM-4:00PM eastern), a heavily traded issue will see its price change several times per second. A stock's price is, for many purposes, considered unchanged outside of market hours.
Thinly traded stocks are handled by specialists who make a market in the stock. Generally, they maintain bid and ask prices based on the most recent sale but factoring in market developments since the last completed sale.