You can view and manage questions and custom fields for your groups by clicking the "Questions" link in the "Start Organizing" menu in the right column. After you choose the group you want to view questions and custom fields for you'll see a list of questions that already have been created for the group up top, and custom fields for the group below.
You can add a new question if you have the permissions to do so by clicking the "Add New Question" button. You can also edit, hide, and delete each question if you have the permissions. Otherwise you can view or hide the questions but not edit or delete them.
Editing a question will change all future questions that are added to forms, but will not change questions that have already been added to forms.
Deleting questions will not delete the underlying data and custom field columns associated with that question, only the question itself.
Hiding the question will remove it as an option when making actions.
If your group is part of a Network, you can also share a question with your child groups.
Adding a question will also create a custom field to store that question's data, if it doesn't already exist. See here for more information on how questions relate to custom fields. You can also create custom fields via the upload process, when editing or adding activists individually, when making blank questions on forms, or over the API. Custom fields do not have to be created first, you can just start using them and they will show up in this list.
However, you can hide custom fields by clicking the hide button to the right in the custom field list. Hidden custom fields will not show up anywhere in the administrative interface, but data can still be stored in them (via questions on forms, for example), and the data that's already in them is not deleted.
You can unhide custom fields and questions by clicking the "View Hidden Items" link at the bottom of the list and clicking "Restore" in the custom fields and questions sections.