The filesystem level explains how ROS files are organized on the hard disk:

As you can see from the preceding diagram, the filesystem in ROS can be categorized mainly as metapackages, packages, package manifest, messages, services, codes, and miscellaneous files. The following is a short description of each component:

• Metapackages: Metapackages group a list of packages for a specific application. For example, in ROS, there is a metapackage called navigation for mobile robot navigation. It can hold the information of related packages and helps install those packages during its own installation.
• Packages: The software in ROS is mainly organized as ROS packages. We can say that ROS packages are the atomic build units of ROS. A package may consist of ROS nodes/processes, datasets, and configuration files, all organized in a single module. 
• Package manifest: Inside every package will be a manifest file called package.xml. This file consists of information such as the name, version, author, license, and dependencies that are required of the package. The package.xml file of a metapackage consists of the names of related packages.

• Messages (msg): ROS communicates by sending ROS messages. The type of message data can be defined inside a file with the .msg extension. These files are called message files. Here, we are going to follow a convention where we put the message files under our_package/msg/message_files.msg.
• Service (srv): One of the computation graph level concepts is services. Similar to ROS messages, the convention is to put service definitions under our_package/srv/service_files.srv.

This sums up the ROS filesystem.