In software development, an Issue or Bug is a glitch in your software system that causes the program to behave in a way other than designed. Issues can range from the show stopper, a fatal error that causes irreparable damage to your software system, to the very minor glitch. Obviously, each issue in the system is not going to have the same importance, or urgency, attached to it. Critical issues, the show stoppers mentioned above, need to be solved first before you can do anything else. Lower urgency issues are the minor ones, which you or your team can put off until time permits. There are tons of other details, that are relevant to the fate of your software, to keep track of like who experienced the issue (was it a customer or someone within the company), when was the bug found, what exactly was it that the user
experienced (in order to be solved, this needs to be extremely detailed!), what solutions were attempted, and much more.

An issue tracking system or tool will manage and maintain your lists of issues. Organizations commonly use Issue tracking systems in their customer support call centers to create, assign, update, track and resolve reported issues. These issues could come from customers directly, or even from the organization's other (non IT) employees. An issue tracking system will often also contain a knowledge base, which information on each customer, fixes for common problems, and other important and useful data. http://www.elementool.com/ebook/IssueTrackingeBook.pdf