WATIR is a browser automation tool that allows testing of web applications through a standardized API across browsers like IE, Firefox, Chrome, and Safari. It started as a way for testers to write Ruby code for testing and has grown to include support for major browsers through implementations that communicate with each browser using different technologies. While WATIR works well overall, further standardization is needed across browser implementations and some browsers like Opera are not yet integrated into the main WATIR codebase.