An Active Record developer primarily works with the Active Record framework in Ruby on Rails to manage relational databases. They create, retrieve, update and delete records in the database, and maintain relationships between different data models. They also implement validations, perform database migrations, and write queries to fetch data. Their role involves understanding and defining business logic, and translating it into code, ensuring data integrity and consistency. They work closely with other developers to integrate the database layer with the application's user interface and functionalities. They also troubleshoot and optimize database performance, and ensure data security.