CFM stands for "ColdFusion Markup." ColdFusion is an API (application programming interface) server developed by Adobe that uses CFML, or ColdFusion Markup Language, to create web applications. Files containing CFML use the CFM extension. Unlike some other server-side languages like PHP, CFML is not open source and is sold commercially, although it is available free to students and teachers.
ColdFusion Markup Language contains the functionality of many server-side languages--the ability to interact with databases and the use of variables and conditional code--but is optimized to create web applications in particular. Like HTML and XML, it is "tag-based," organizing code inside a series of identifiers known as tags. Because of the similarities in appearance to HTML, ColdFusion is a good entry point for web developers from a static HTML background who want to break into building applications and dynamic pages.
Since CFM files consist only of simple text, CFM files can be opened with any text editor. However, you must have knowledge of CFML to understand the code, and the file cannot be used unless you have an application server that reads CFM files--usually ColdFusion itself, but also competing products like Railo, BlueDragon, Corel Web Developer and IgniteFusion, among others. CFML can also be integrated with the Java programming language and Microsoft's NET software framework.