Custom fields are custom visitor properties that can be used to collect or set additional information onto a visitor's profile.
There are 7 types of custom fields:
A simple one-line text field.
A multi-line text field.
An email text field. This will have the type set to email in HTML and will only allow email format values.
A numeric text field.
A checkbox field that can have just two possible values - true and false.
A radio input list, that can have multiple options out of which only one can be selected.
A select dropdown that can have multiple options out of which only one can be selected.
You cannot change a field type after it was created.