Where do you define the django forms?

Probably some simple question, but does it say in Django's homepage where you should define the forms? Does the code goes into the model.py? But that file should only consist of Django's database models right? So I wonder where should the forms go into?

Forms that I mean is this one you import

from django import forms

and then create for example a form like this (code is taken from Django's homepage)

class ContactForm(forms.Form):
    subject   = forms.CharField(max_length=100)
    message   = forms.CharField()
    sender    = forms.EmailField()
    cc_myself = forms.BooleanField(required=False)