Override Django User Model

I'm writing a Django app. I wanted to override the Django User Model for the following reasons:

1) I want to use LDAP for authentication. Therefore, Django authentication is not necessary for me.

2) I already have a user table and I want to reuse that table. I do not want Django to create a dup table for me. Also the table structure doesn't "fit" Django User model very well.

Therefore, I'd like to override the Django User class.

Does anyone have any good example good that I can learn from?