Turck MMCache is a opcode cache for PHP. It increases performance of PHP scripts by caching them in compiled state, so that the overhead of compiling is almost completely eliminated. Also it uses some optimizations for speed up of PHP scripts execution. Turck MMCache typically reduces server load and increases the speed of your PHP code by 1-10 times.
Looks like the same thing as PHP Accelerator. Hopefully benchmarks and comparisions will come out soon.