Controlling risk is one of the most important ingredients of successful trading. While it is emotionally more appealing to focus on the upside of trading, every trader should know precisely how much he is willing to lose on each trade before cutting losses, and how much he is willing to lose in his account before ceasing trading and re-evaluating.
Risk will essentially be controlled in two ways: 1) by exiting losing trades before losses exceed your pre-determined maximum tolerance (or "cutting losses"), and 2) by limiting the "leverage" or position size you trade for a given account size