Jira is a proprietary issue-tracking system. It can be used to track bugs, resolve issues, and manage project functions. There are many tools available on the market, but the best thing about Jira is that it can be easily customized and configured to suit the specific needs of an organisation and there are plenty of ways to add more features.
Out-of-the-box, Jira offers defect/bug tracking functionalities, but it can also be customized to act like a help desk system, a simple test management suite, or a project management system with end-to-end traceability for software development projects.
Jira is mainly accessed using a web browser but it has lot of integrations with other tools and Jira comes with a RESTful API so you can interact with it programmatically.