Forms are used to collect information from users. You can mark any specific question in form as User Attribute. For example you are asking for favorite color in form from the user so you can mark this question to store as User Attribute, so whenever user would answer this question his/her answer would be stored separately, you can use User Attribute to create User Segments. If you don’t know about User Segmentation Read this article User Segmentation.
Here is the process to store form field as User Attribute
Open forms from the left side menu
Give name to your form and click next
Now define all the question you want to ask from your user. To mark any question as User Attribute, hover over the form field, a Field Setting Panel would appear over the right side, click on the setting icon on the field setting box, as you click on the setting icon a form field area would be extended containing advance option for the field. Here is the option to mark that field as User Attribute. Click on User Attribute Checkbox to store that field as User Attribute