Inherit OpenERP Models:

1-Object Inheritance -
Objects may be inherited in some custom or specific modules. It is better to inherit an object to add/modify some fields. This inheritance can be done by two possible ways and both the ways result in a new class of data, which holds parent fields and behavior as well as additional fields and behavior but they differ in heavy programmatical consequences.

Example- inherit account module object in openerp

from osv import fields, osv
class invoice_order_test(osv.osv):
    _name='account.invoice'
    _inherit='account.invoice'
    _columns={
           'cartage':fields.float('Cartage',readonly=False),
           'credit_note':fields.text('Credit Note',readonly=True),
    }

2-view inheritance:-
In openerp you have to inherit only view form of account module and add some new field
in account module.

Example- inherit account module view in openerp

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
   <data>
        <record id="invoice_form" model="ir.ui.view">
        <field name="name">account.invoice.form.inherit</field>
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_form" />
        <field name="arch" type="xml">

        <field name="amount_untaxed" position="after">
        <field name="cartage"/>
        <field name="credit_note"/>
        </field>
        </field>
      </record>
    </data>
</openerp>