ACL stands for access control List in Joomla. ACL is a mechanism using which we can provide access to the users to access the articles,modules, plugins and other extension. It's the feature of Joomla using which we can publish an article, module, plugin and other extension to a particular user group.

To understand the ACL in more detail let's take an example. Suppose we have a module which we want to display only a particular user group. For performing this task we need to follow the steps given below:

1. Login to the Backend(administrator) of Joomla.
2. Go to Users -> Groups -> Add new Group
3. A form will open to add the Group with Group Title and Parent. You can select the parent according to the your requirement. Save & Close it.
4. After that again click Users -> Access Levels -> Add New Access Level.
5. A form will open where we will see the Level Title and a list of user groups. Here, we will see the name of the group which we have created in previous steps. Enter the Level Title and select user group which you have created or according to you requirement. Now click on the button Save & Close.

Above steps will create a Access Level. Now our next step to assign the user group to all the users whom we want to display the module. For this we need to follow the steps given below:

6. Go to Users -> Manage(in admin section of Joomla)
7. Now we click on the name of any one user whom we want to display the module or need to assign the user group i.e., edit the user details.
8. Select the Tab Assigned User Groups. Here we will find all the groups name. We will select the user group name which we have created in previous steps. We can assign more then 1 group to a user. Click on Save & Close button.
We will repeat step 8 for all the users whom we want to display the module.

Now our next step is to display the module only to this particular user group. For this we will follow the steps given below:

9. Go to the Extensions -> Modules(in admin section of Joomla)
10. Now we will select and open the module which will be displayed to a user group. Here we will find a field Access. We will select the access level created in earlier steps. Click on Save & Close button.

Now the above step will assign the module only to that particular user group. Other users will not able to see the module. Follow the steps given above and you will see the ACL magic.