Most sources define a trader as an individual who engages in the buying and selling of assets in financial markets (stocks, bonds, derivatives, currencies, etc) either for him/herself or on behalf of another person or institution. The most imporatnt feature that differentiate a trader from an investor is the duration of such operations; Investors tend to hold for a longer-term, while traders tend to hold assets for much shorter periods and capitalize within short-term trends.

A good trader should have a combination of skills broadly speacking quantitative, qualitative and behavioral qualities but if you must highlight one should be Risk Managemnetin which they must constantly monitor their current and potential positions to ensure that the risks they take are optimal.