Gevurah is the fifth of the ten sefirot (not counting the "hidden" sefira Daath), and second of the "emotive" attributes in Creation.
Wikipedia: "Gevurah is usually understood as God's mode of punishing the wicked and judging humanity in general. It is the foundation of stringency, absolute adherence to the letter of the law, and strict meting out of justice. This stands in constrast to Chesed.
Gevurah is associated in the soul with the power to restrain one's innate urge to bestow goodness upon others, when the recipient of the good is judged to be unworthy and liable to misuse it. As the force which measures and assesses the worthiness of Creation, Gevurah is also referred to in Kabbalah as midat hadin ("the attribute of judgment"). It is the restraining might of Gevurah which allows one to overcome his enemies, be they without or from within (his evil inclination).
Chesed and Gevurah act together to create an inner balance in the soul's approach to the outside world. While the "right arm" of Chesed operates to draw others near, the "left arm" of Gevurah reserves the option of repelling those deemed undeserving. (Even towards those to whom one's initial relation is that of "the left arm repels," one must susequently apply the complementary principle of "the right arm draws near)."