With Sharing – Enforce the sharing rules that apply to current user.

Without Sharing – Doesn’t enforce the sharing rules.

If a class is not declared as either with or without sharing, the current sharing rules remain in effect. This means that if the class is called by a class that has sharing enforced, then sharing is enforced for the called class.

public with sharing class CWith 
{
  // All code in this class operates with enforced sharing rules. 
    

  Account a = [SELECT . . . ];

  public void m() 
  { 
      . . . 
  }
  
}

public without sharing class CWithout 
{
  // All code in this class ignores sharing rules and operates  
    
  // as if the context user has the Modify All Data permission. 
    
  public void m() 
  {  
     . . . 

    // This call into CWith operates with enforced sharing rules 
    
    // for the context user. When the call finishes, the code execution  
    
    // returns to without sharing mode. 
    
    CWith.m();
  }


  public class CInner 
  {
    // All code in this class executes with the same sharing context 
    
    // as the code that calls it.  
    
    // Inner classes are separate from outer classes. 
    
    . . .

    // Again, this call into CWith operates with enforced sharing rules 
    
    // for the context user, regardless of the class that initially called this inner class. 
    
    // When the call finishes, the code execution returns to the sharing mode that was used to call this inner class. 
    
    CWith.m();
  }

  public class CInnerWithOut exends CWithout 
  {
    // All code in this class ignores sharing rules because 
    
    // this class extends a parent class that ignores sharing rules. 
    
  }
}