Day Trading essentially involves closing every position before the end of the trading day. If you buy 10 shares of Microsoft (MSFT) and sell them again before the end of the day then you are a day trader. Day Traders come and go flat which means that they both start and finish the day with no investments. The key to success is to buy low and sell high often enough to cover your costs.
Keep in mind that most Day Traders go broke and quit within the year. Remember also that most Day Traders are frequent traders; whether the Day Trader makes or loses money the broker still gets his commission.