A jQuery plugins developer designs and implements reusable modules that add to the functionality of jQuery, a popular JavaScript library. They write code that manipulates the Document Object Model (DOM), handles events, creates animations, or adds other interactive features to web pages. Their responsibilities also involve debugging and optimizing plugins for performance and compatibility across different browsers. They need to ensure that their plugins are easy to use and customizable, often providing documentation for other developers. They must stay updated with the latest web standards and technologies, and should have strong skills in JavaScript and HTML/CSS.