This is a register architecture machine emulator. The full specification can be found here.
Here is the full spec, so you don't even need to follow the link:
Okay, but how do I use this?
Write your program in the leftmost box you see.
Press LOAD to actually load the program. You should see the instructions you've just typed on the right now.
Press RUN to execute the whole program until it halts.
Press RUN SINGLE STEP to execute just one instruction.
Reset will... reset everything. No program, no input, no output.