Introduction - If you have any usage issues, please Google them yourself
RRT (Rapidly-Exploring Random Tree) is a sampling-based algorithm for solving path planning problem. RRT provides feasable solution if time of RRT tends to infinity.
RRT* is a sampling-based algorithm for solving motion planning problem, which is an probabilistically optimal variant of RRT. RRT* converges to the optimal solution asymptotically.
RRT*FN is a sampling-based algorithm based on RRT*. RRT*FN inherents asymptotical convergence to the optimal solution, however RRT*FN implements it using less memory.