InfoPath is FrontPage for XML documents.

It serves as a wrapper for BizTalk (thereby actually making BizTalk "necessary").

Sharepoint Server reads the properties of InfoPath documents, but these "properties" are actually the XML nodes of the document itself (thereby finally making document properties useful in Office).

InfoPath can merge multiple XML documents of the same schema into one document in an "aggregation scenario." This functionality can be called from SharePoint.

InfoPath documents can be saved to HTML (or MHTML) but these serve only to "display" the form. The HTML is read only: all of the data-entry and interactivity is meant to be available only to InfoPath users. This implies that cross-vendor or cross-platform interoperability is confined to the XML data itself---not user-interface interactivity.