Custom profile fields are an incredibly powerful feature to expand what you know about each member. During onboarding, custom profile fields can be included as a set of prompts to ensure you know the most about every new member that joins.
Standard Profile Fields
Standard profile fields are those that are always captured and displayed on profiles. You cannot have a profile in your community without capturing email - so the email field is always captured. You can toggle some standard profile fields on/off to remove.
Custom profile fields
Custom profile fields can be added. Each custom profile field can have a different input type, such as Single Select, Multiple Select, Written Answer or Link.
For example, you may want to capture each Members level of education. You would create a custom profile field called "Education" and then choose "multiple select" as the input type. From there, you can define standard answers they must choose from (eg. high school, college).