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# Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. // SPDX-License-Identifier: CC-BY-SA-4.0
title: ROS basic concepts
permalink: /modules/1/ros-basic-concepts.html
---

In this module, we will discover the ROS basic concepts, which will allow you to write software in ROS.


## ROS core elements
A best practice in ROS is to design software as a collection of small programs. A communication mechanism between programs is necessary and is provided by ROS. The main communication approach in ROS is based on **publish/subscribe** message passing. Publish/subscribe is defined in software engineering as a pattern where publishers send messages without knowing who are the recipients; and subscribers receive messages from publishers based on their subscription to information of interest.

To enable this communication, there are four main ROS core elements:
1. **ROS master** -- a process to facilitate the communication between robotics programs.
2. **ROS nodes** -- running instance of a ROS program implementing some robotics components.
3. **ROS topics** -- named buses over which nodes exchange typed messages.
4. **ROS messages** -- contains the information.

The following video graphically shows what happens when a node is publishing velocity commands, and another node is subscribing to the related ROS topic to receive those velocity commands.

{% include video-file.html url="/img/ros-master" %}

Only one ROS master should run in a system. The order of the nodes registration to the ROS master is not guaranteed in general. ROS defines a number of standardized typed messages which can be found at the [common messages page](http://wiki.ros.org/common_msgs). 

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With an understanding of the message passing mechanism, let's see how we can move the robot in the [next unit]({{ site.baseurl }}{% link _modules/mod-1b-master-nodes.md %}).


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