Why does some script open higher or lower than previous day

Why does some script open higher or lower than previous day and what is the possible trend if it open higher or lower than previous day

Markets close at 3.30pm and opens again next day at 9.15 am. That is 17+ hours when the market is closed. Every second when the market is open, demand/supply based on people perception moves the stock price up or down.

When the market is closed for 17+ hours, imagine how much the perception could have changed. So every day when the market opens, there is price discovery happens based on demand/supply. Since the world has changed during the 17 hours, so has the perception, and hence the opening price of the day.

If you follow technical analysis, it is best to be with the trend. So if it opens up, be bullish, and if it opens lower bearish. Check this: http://zerodha.com/varsity/, good place to get started learning markets.