A Heroku developer designs, builds, and manages applications hosted on the Heroku platform. They use Heroku's platform as a service (PaaS) to quickly deploy, manage, and scale applications. This includes writing code in a variety of supported languages such as Python, Java, and Ruby, setting up databases, and implementing Heroku's data services. They also monitor app performance and troubleshoot any issues using Heroku's tools. Furthermore, they manage user access, ensure security compliance, and integrate third-party services. Essentially, they leverage Heroku’s cloud platform to build robust, scalable, and efficient applications.