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 models
from django.db.models import signals

def 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