In Joomla 1.5 version, other than the basic fields in the user registration form, any additional fields can be added only programatically. Refer link below for more information
But this feature is made simple in Joomla 2.5 upgrade by just turning on a field in the administrator area. Install 2.5 joomla and go to admin->plugin and search for "profile" in the search box. There will be a plugin called "User - Profile", enable it.Now go to the front page and click new user registration. You could see more fields added to the default fields.This is because of profile plugin developed according to the following link. http://docs.joomla.org/Creating_a_profile_plugin. The code is available in the following location..[root]/plugin/user/profile/ Have a look into the [root]/plugin/user/profile/profiles/profile.xml where we can see some form elements(joomla jform). The ACL concept in Joomla can be used to provide different levels of access to different types of administrators who work in the back end activities of the system. The super administrator is responsible for creating this access control on other administrators . ACL concept cannot be extended to create access controls for the site users (viewers)
Lets understand the concept of ACL using a scenario.
There is an e-commerce website designed for an apparel stores catering to women, men and kids garments. The site provides an option where a registered user, suppose is a female ,can view only women apparel, and similar access views for men and kids to see only their category of apparels.
This function cannot be provided by using the ACL concept in Joomla. This is because the ACL can be used in the administrator area in the back-end. The super administrator gives the access to add or delete products to a set of administrators while they are denied accessing the customer sensitive data like credit card details, whose accessing permission lies only with the super administrator.