A GIS (Geographic Information System) application developer designs, tests, and implements GIS and mapping software. They create applications to visualize, analyze, and interpret geographical data for use in various industries like urban planning, transportation, or environmental science. They work with geospatial data, databases, and scripting languages to develop software that can process and display this data in a meaningful way. Additionally, they collaborate with other team members to understand user needs, troubleshoot technical issues, and ensure the application's performance and reliability. They also keep abreast of new technologies to improve existing applications or develop new ones.