As the term describes, a daytrader is a stock trader that buys and sells, usually on more than one position, only throughout the course of a single trading day. Daytraders live in 1-minute, 5-minute, and 15-minute time charts, typically. Their goal, just like any trader, is to buy low and sell high, but the difference is that they focus on the minute-by-minute fluxuations of a stock price.
They are looking to earn smaller gains, many times per day. By doing so, that can add up to a substantial profit by days end.
The daytrader can capture many micro-trend changes - changes that would be unprofitable to a swing trader, for example. Since daytraders tend to invest with very small amounts of their portfolio at a time, all the while spreading out that limited exposure across many different trades, it can help reduce risk dramatically.
The very reason why daytraders want to trade in a minute timeframe is the same reason why it can be dangerous to do so. Trading throughout the day can be much more volatile then longer time frames - moves during the day, specially from pre-trading to open, can be very active. This can also setup the daytrader to incur largers losses.
A daytrader needs to have a substantial bankroll to last over the long-term. As mentioned, daytraders only trade with 1-5% of their entire portfolio at a time. Well, if the trader wants to make any kind of substantial profit, then they have to have a large enough portfolio to enable their 1-5% trades to earn enough to be worth the risk they take on to do so.
Another big downside to daytrading is the time you spend doing it. You literally have to be watching the market constantly, looking for the smallest opportunity to profit - this can wear on a person in the long-term.
Daytrading became very popular, in particular, during the amazing rise (and fall) of the Dot Com Tech bubble in the late 1990's-early 2000's. Typical to a bubble, such as the great stock market crash of 1929, everyone thinks they can make an easy profit just by following the leads of the herd.
For daytrading to be profitable and worth doing, the market has to be on the move - extreme volatility is the conditions that the trader needs, but is also the exact conditions that can burn the trader if they are wrong - it's a double-edge sword in the world of daytrading.