The United States Bishops have created a resource page on their website dedicated to explaining the difference between these two feet of social action. You can visit the page here.
Works of Mercy, or Charity, are those actions that meet the immediate needs of those we serve. They often involve direct service – food pantries, clothing drives, soup kitchens, shelters – and help people survive from day-to-day.
Works of Advocacy, or Justice, are those actions that identify root causes of injustice – why someone is hungry or homeless – and work to eliminate them. Works of Advocacy address structural policies and laws that create obstacles for people trying to better their lives.