Think of custom fields as a place where you can store relevant data that doesn't fit into a standard grouping system. Custom fields can be found at the top of a workspace page:
Or in the right hand column:
Edit these fields by using the "Edit" link that appears when rolling over the right hand column:
As a site administrator, you can change the names of the custom fields in the Admin options area:
Create new custom field groups, fields, and reorder them:
You can also have important custom fields show up at the top of the page:
Advanced options are found under "More:"