Luke W has posted a great series discussing the use of Forms in Web applications. The first, Web Application Form Layout, introduces the subject, focusing upon consistent and usable presentation of the ubiquitous Web form. The second, Web Application Form Design provides further study of various layout options and their impact on usability and user-understanding. The third, Web Application Form Design Expanded provides further clarification and examples of unifying the various components of Web forms.
I highly recommend tat every Web developer and designer read through the articles. With an understanding of these concepts, the Web professional will produce better, more-usable Web forms.