The two feet of social justice are called charity and justice. Charity refers to social service and performing direct services for those who are less fortunate, such as food, clothing, and shelter. 
This work of mercy, the formal term for this foot of social justice as recognized by the Catholic Church, is a temporary, day to day response to the needs of others. Charity also responds to immediate needs and is directed at the effects of injustice. Giving a hungry man food is an example of a work of mercy as a temporary response to this need.
Justice refers to social change and the promotion of social change in institutions or political structures. This social action, the formal term of this foot of social justice as recognized by the Catholic Church, responds to long terms needs and is directed at the root causes of social problems. 
It alleviates at the root of the problem and addresses the underlying structures or causes to these problems. Teaching a man a skill to help him feed himself and his family for the future as a more permanent solution to this problem.