Who were the kulaks?
The kulaks were Russian peasant landowners particularly famous (unfortunately) for being the targets of many murderous purges during Stalin's regime.
Kulaks were particularly persecuted by Stalin as responsible for undermining, with their protests and opposition, the agricultural plans for the agriculture of the central Soviet government.
The kulaks were perfect targets for the regime also because they owned small plots of land and were employing other poorer peasant under them (making them similar to the old Tzarist's greedy and cruel landowners).
Kulaks were purged and murdered in great number (in Ukraine, for example) by deportation and starvation under the Soviet campaign of dekulakization.