According to the U.S. Department of Health and Human Services:
Isolation separates sick people with a contagious disease from people who are not sick. Quarantine separates and restricts the movement of people who were exposed to a contagious disease to see if they become sick. These people may have been exposed to a disease and do not know it, or they may have the disease but do not show symptoms.
You can learn more about quarantine and isolation on the CDC website.