An NBM file is a NetBeans module packaged for delivery via the web. The principal differences between it and a module jar are:

  • It is compressed
  • It can contain more than one JAR file - modules can package any libraries they use into their NBM
  • It contains metadata NetBeans will use to display information about it in the update center, such as the manifest contents, the license, etc.
  • NBMs are typically signed for security purposes

NBM files are just zip files with a special extension, which use the JDK's mechanism for signing jars. Unless you're doing something unusual, you will not need to worry about the contents of NBMs - just let the standard Ant task for NBM creation take care of it for you.

Source: NetBeans FAQ