A Debian developer contributes to the Debian Project, an association of individuals who have made common cause to create a free operating system. Their tasks include maintaining software packages, handling bug reports, testing for software stability, assisting with infrastructure, and improving the OS. They also participate in decision-making processes, vote on proposals, and help shape the future of the Debian Project. They collaborate with other developers, both in Debian and upstream, and interact with users to understand their needs. Their ultimate goal is to ensure that the Debian OS remains high-quality, free, and meets user expectations.