What is day trading? Simply put, it is the process by which you buy, sell and short stocks and end the day with a flat postion. It is the opposite of position trading, where you buy or short stocks with the intention of holding them for an extended period of time. This could be overnight, or possibly months or years in duration.
There are a variety of techniques that can be used, among them trading on news before the open, intraday or after the close, momentum trading, fading the open, sector trading, technical trading and so on. Each individual trader, through trial and error determines what their unique strength is, as well as what is currently working in the market. Many times by the time you determine what's working, it no longer works.
There are a variety of tools that are used, some as simple as stocks in the news in Investors Business Daily. A stock quote service that includes fast, inexpensive and reliable order entry is critical. Tools such as Instinet or other 3rd market sources is equally critical (these third markets trade before, during and after the exchange hours. Pre and post-market they can be thin, particularly on listed stocks with wide bid/ask spreads.). A charting service in which you can insert moving averages suh as the 200-day is also important.
Each of these methods will be discussed, but some of the key concepts to remember are that the trend is your friend (or don't fight the tape), maintaining controls and disciplines (not letting losers run), and buying dips and shorting rallies (nothing goes up and down in a straight line). The bottom line motto to remember is that bulls make money, bears make money and pigs get slaughtered.