LeJOS is a virtual machine, class library, and tools for writing Java code for LEGO Mindstorms. LMS is software to simulate LeJOS programs with 3D models. Using LeJOS and LMS allows you to do Mindstorms programming even if you don't have the actual Mindstorms kit. You write your programs with LeJOS, and then run them in the LMS simulator. If you do have Mindstorms, the LeJOS system can be downloaded to the Mindstorms "brick" to run your LeJOS programs on the Mindstorms hardware!