What are CFX tags?

ColdFusion Extension (CFX) tags are custom tags written against the ColdFusion Extension Application Programming Interface. Generally, you create a CFX tag if you want to do something that is not possible in CFML, or if you want to improve the performance of a repetitive task.
One common use of CFX tags is to incorporate existing application functionality into a ColdFusion application. That means if you already have the code available, CFX tags make it easy to use it in your application. 
CFX tags can do the following:

• Handle any number of custom attributes.
• Use and manipulate ColdFusion queries for custom formatting.
• Generate ColdFusion queries for interfacing with non-ODBC based information sources.
• Dynamically generate HTML to return to the client.
• Set variables within the ColdFusion application page from which they are called.
• Throw exceptions that result in standard ColdFusion error messages.
  You can build CFX tags using C++ or Java.