In the List tab, click on the 
button of a list, then select Custom fields.
Here are the different types of fields:
- Text field: short text area for few words
- Text area: big text area
- Date: in the yyyy-mm-dd format, for birthday dates for example
- List: with different options
- Radio button: only one choice is possible
- Check box: multi choices are possible
- Hidden field: this field will not be visible for the subscriber but it can contain a value