Cascading means that if you insert, update or delete an object, related objects are inserted, updated or deleted as well. If you do not use cascade you would have to save both objects independently. If you initially create objects and you do not cascade then you must save each object explicitly.

Department d = new Department();
Team t1 = new Team();
Team t2 = new Team();
d.getTeams().add(t1);
d.getTeams().add(t2);
t1.setDepartment(d);
t2.setDepartment(d);
session.save(d);
session.save(t1);
session.save(t1);

If you configure cascade on the department side

Annotation. 

@OneToMany(cascade = {CascadeType.ALL})
private Set<Team> teams;

XML. 

<set name="teams" table="tteam" inverse="false" cascade="all">
  <key column="department_id"></key>
  <one-to-many class="Team"/>
</set>

then you only need to call

session.save(d);

and the rest will be automatically cascaded. You can combine options, as well.

JPA Standard Annotation. 

@OneToMany(cascade = {CascadeType.MERGE, CascadeType.PERSIST})

Hibernate Annotation. 

@OneToMany
@Cascade({CascadeType.MERGE, CascadeType.PERSIST})

XML. 

<set name="teams" table="tteam" inverse="false"
  cascade="persist,lock,replicate,save-update,delete,delete-orphan,refresh">

The following tables explains the different cascade types available. You will see that they are linked to methods from the session like session.persist(), session.delete(), session.buildLockRequest(LockOptions.NONE).lock(), ….

Orphan Removal

If you remove for example an invoice position from the collection of an invoice, it is called an orphan entit. If you configure orphan removal it will be deleted just by fact that it was removed from the collection.

Since Java Persistence 2.0 orphan removal, the Hibernate cascade type is deprecated. Here is the correct way to use it.

@OneToMany (cascade = CascadeType.PERSIST, orphanRemoval = true)
private List<InvoicePosition> positions;

Usage. 

Invoice invoice = (Invoice) session.get(Invoice.class, 4711);
invoice.getPositions().remove(1);