CDA (Clinical Document Architecture) is an XML-based syntax that defines the structure of certain medical documents like Continuity of Care Documents (CCD) and imaging reports. It’s meant to create a standard way for computers to exchange these documents while still being human readable through a web browser (like other XML documents).
Basically, CDA is to your medical documents as HTML is to web pages: It’s the structure that’s used in order for it to work with any browser/system/etc.