The differences between Acrobat Forms (AcroForms) and XML Forms Architecture (XFA) also know as LiveCycle forms are as follows:

AcroForms are the original PDF forms technology, first introduced in 1998. AcroForms accept input in both Forms Data Format (FDF) and XML Forms Data Format (XFDF). Many 3rd party vendors support AcroForms.

Adobe’s acquisition of Accelio in 2003 brought that company’s XFA forms technology to Adobe, where it is made available via Acrobat LiveCycle Designer. XFA forms are inherently XML rather than PDF, and are incompatible with conventional PDF files. XFA forms “break” Adobe’s Acrobat software, in the sense that Acrobat cannot be used to modify a LiveCycle Designer-created document. XFA forms are not yet fully supported by third party PDF viewers’ vendors. Currently Adobe LiveCycle server software is required to process XFA forms in a server environment.

AcroForms have capabilities not found in XFA and conversely XFA has some capabilities not found in AcroForms.  For example:

• AcroForms support the concept of “Templates”, allowing additional pages to be added to the PDF form document to support populating the form with multiple database records.
• XFA supports the concept of document reflow allowing a field to resize if needed to accommodate data.

For more information on the specific differences between AcroForms and XFA forms, see the original Appligent white paper: A Quick Introduction to Acrobat Forms Technology (PDF).

Like XML, an FDF file is a structured text file. And like XML, FDF can be easily created.  Simple examples of both FDF and XFDF can be found here: Forms Data Format.

Forms Data Format is part of the international standard for the Portable Document Format (PDF) and is fully documented in ISO-32000 “Document management — Portable document format — Part 1: PDF 1.7”