At its core, Spring is a DI framework. Before we get started with understanding Spring, we need to understand the concepts of dependency and DI. 

Object-oriented applications are built around objects and their interaction with other objects. Typical applications involve thousands of objects interacting with each other.

Let's consider an example class, BusinessServiceImpl, as follows:

    public class BusinessServiceImpl { 
      public long calculateSum(User user) { 
        DataServiceImpl dataService = new DataServiceImpl(); 
        long sum = 0; 
        for (Data data : dataService.retrieveData(user)) { 
          sum += data.getValue(); 
        } 
        return sum; 
      }
    }

BusinessServiceImpl creates an instance of DataServiceImpl and uses it to get the data from the database.  DataServiceImpl is a dependency of BusinessServiceImpl.

Dependencies of a class are the other classes it depends on to fulfill its responsibilities.