In Most of the cases we have to create the custom permissions based on our requirement like "No Delete" and also to restrict the users to "create sub sites".
Already we having default groups like "Full Control","Contribute" but we need the same contribute with out delete in this case we have to create custom permissions.
First we have to create a permission level at site collection level as per the below steps.
Navigate to ->Site Settings->Site Permissions->Top Ribbon Click on Permissions Levels
Here we can able to create by clicking on the "Add a Permission Level" as shown below
We can able to check the Permissions based on our requirement example i am selecting "Select All"
Click on Create.
Now we able to see the our newly created custom permissions.
Now we can add the users in to our newly created custom group permissions.