Explain Objective C retain cycle with real world example?

I am reading about retain cycle that, "A retain cycle can take a few forms, but it typically means that object A retains object B, and object B retains object A, but nothing else retains object A or B". But I'm not clear about that concepts. Please can anyone explain retain cycle with real world example.

Thanks.

A simple example,a person lives in a department,a department have one person(Suppose have one)

@class Department;

@interface Person:NSObject
@property (strong,nonatomic)Department * department;
@end

@implementation Person
-(void)dealloc{
    NSLog(@"dealloc person");
}

@end
@interface Department: NSObject
@property (strong,nonatomic)Person * person;
@end

@implementation Department
-(void)dealloc{
    NSLog(@"dealloc Department");
}
@end

Then call it like this

- (void)viewDidLoad {
    [super viewDidLoad];
    Person * person = [[Person alloc] init];
    Department * department = [[Department alloc] init];
    person.department = department;
    department.person = person;
}

You will not see dealloc log,this is the retain circle