Suppose you want to define a user permission to provide user to create new items and update existing items but prevents to delete anything. Unfortunately, the standart sharepoint permissions does not supply this kind of permission. But you can create custom permissions at site collection level.
Open Sharepoint with the admin user. Navigate to Site Settings > Modify all Site settings (above)
Navigate to Advanced permissions (above)
Now navigate Settings > Permission Levels
In Permission Levels Screen, you can see all the permission levels that are defined in this Site Collection
To Create a new one, Simply click to Add a Permission Level
For Example if you want to create someone that can add new items, create alerts (for himself) but can not change existing items and delete anything you should create something like that: