In some cases you need a permission level like the user needs to add an Item but cannot edit or delete an Item. In this case you cannot use neither contribute (which will give permission to edit/delete) or Read permission level (which will not give permission to add even). So in such case you need to create a customized permission level.
This can be done as follows.
- Go to Site Actions -> Site Settings ->Advanced Permissions
- In this list select Settings -> Permission Levels. Here you can see all the permission levels available for the current site.
- Click Add a Permisssion Level.
- Now create your own customized permission by selecting the various permissions displayed there.
- Now when ever you create a group or a user in the permissons column, you will be able to see the permission level you created.