You can find very good content about django signals over Internet by doing very small research.

Here i will explain you very brief about Django signals.
What are Django signals?
Signals allow certain senders to notify a set of receivers that some action has taken place

Actions :

model's save() method is called.
django.db.models.signals.pre_save | post_save

model's delete() method is called.
django.db.models.signals.pre_delete | post_delete

ManyToManyField on a model is changed.
django.db.models.signals.m2m_changed

Django starts or finishes an HTTP request.
django.core.signals.request_started | request_finished

All signals are django.dispatch.Signal instances.

very basic example :

models.py

from django.db import modelsfrom django.db.models import signalsdef create_customer(sender, instance, created, **kwargs):    print "Save is called"class Customer(models.Model):    name = models.CharField(max_length=16)    description = models.CharField(max_length=32)signals.post_save.connect(receiver=create_customer, sender=Customer)

Shell

In [1]: obj = Customer(name='foo', description='foo in detail')In [2]: obj.save()Save is called

Apart from the explanation given by Prashant, you can also use receiver decorator present in django.dispatch module.

e.g.

from django.db import modelsfrom django.db.models import signalsfrom django.dispatch import receiverclass Customer(models.Model):    name = models.CharField(max_length=16)    description = models.CharField(max_length=32)@receiver(signals.pre_save, sender=Customer)def create_customer(sender, instance, created, **kwargs):    print "customer created"

For more information, refer to this link.

In the signals.post_save.connect(receiver=create_customer, sender=Customer)... sender will always be the model which we are defining... or we can use the User as well in the sender.