Custom text between webform elements
Assuming you are talking about the Webform module, you can create an element with type of "markup".  There are a few options for this, including input format.  If you have everything configured properly, you can use a WYSIWYG editor on the fields, too.

If you are using the Form API, then you can use #type = "markup"; to put pretty much whatever HTML you want in your custom form.