Automated (gui based) testing tool from Rational Corporation.
Testing tool... BigBangTesting? ProgrammerTest or CustomerTest testing? GuiTesting?
All of the above. Any kind of test that requires (or supports) a predetermined set of user input events can be executed using a tool like this.
How lightweight is this tool? Would writing unit tests with Robot have a similar feel to using, say, jUnit?
Robot is neither lightweight in usability or in price. This tool will set you back a few $K. Robot is a development environment with its own custom scripting language, SQABasic. JUnit is easy to use if you know Java. Similarly, Robot could be easy to use if you know SQABasic. SQABasic is designed to hammer a GUI.
GUI-based test tools, like WinRunner and Robot, lead to many to create brittle test suites that break as soon as the GUI changes or the navigation through the GUI changes. See Carl Nagle's white paper on automated testing frameworks(http://safsdev.sourceforge.net/FRAMESDataDrivenTestAutomationFrameworks.htm) for the lowdown.