An Apex Data Loader developer is responsible for importing, exporting, and deleting large quantities of data in Salesforce. They use the Apex Data Loader tool to manage these operations, ensuring that data is transferred accurately and efficiently. Their duties also include creating mappings between source and target fields, handling CSV files, and resolving errors during data operations. They often work with complex data structures, requiring a strong understanding of Salesforce object models. Additionally, they may also need to automate these processes, necessitating knowledge of scripting and scheduling tasks. Their ultimate goal is to facilitate seamless data management in Salesforce.