Atlas vs. Chef, Puppet, Ansible and Other Configuration Management
Configuration management tools install and manage software on a machine that already exists. Depending on how you use Atlas, configuration management is still required.
Sometimes, configuration management systems launch and manage infrastructure. In this case, Atlas replaces that functionality by launching images with Terraform. This immutable approach can speed up deploy times, increase visibility into what is deployed and help guarantee safety in deployments. "Immutable infrastructure", as it is sometimes called, has numerous other benefits that are written about broadly.
The Atlas workflow focuses on immutability in deployment, taking the view that infrastructure should be launched in a configured state rather then being configured after being launched.
When using Atlas to deploy images as covered in the continuous deployment use case, Atlas expects configuration to be applied at build-time, during Packer builds. Packer can leverage many different configuration management tools to configure the image. It's often the case that Puppet, Ansible or Chef are used to configure the image, but once configured the image is deployed and managed by Terraform.
Because of this, Atlas can use existing configuration management systems. Moving to image deploys is then just a matter of configuring Packer and Terraform.