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