There are numerous ways to setup multiple Magento stores that all share the same codebase and backend, but what method you use depends on your needs.
This article is written with cPanel in mind, though the methodologies listed below apply no matter what control panel you’re using.
Jump To Section

URL Structure
Shared Hosting Caveat
Adding Another Store In Magento
Parked Domain Method
Addon Domain Method
Subdomain Method
Subdirectory Method
Managing Multiple Stores
Secure Checkout For Each Domain

URL Structure
The actual URL structure of your stores is a matter of personal preference. You can, for example, have two entirely different stores running …

Site Management

Control multiple websites and stores from one Administration Panel with ability to share as much or as little information as needed
Multilingual
Support for Localisation
Support for multiple currencies
Administration Permission System Roles and Users
Web Services API for easy integration between Magento and any third-party application
Flexible tax rate management with support for US and International markets
Fully 100% customisable design using templates
Customer Groups
One-Click Upgrades
Content Management System for Informational Pages

Mobile Commerce

iPhone Optimised

Marketing Promotions and Tools

Flexible Coupons (pricing rules) with ability to restrict to stores, customer groups, time period, products, and …

Magento Frontend: http://asia-connect.com.vn/demo/magento
Magento Backend: http://asia-connect.com.vn/demo/magento/admin
Admin: admindemo/admindemo