Joel Jamieson wrote an excellent piece on the subject of cardio training for MMA.
The LSD vs. HIIT Debate
What is LSD? - it stands for Long Slow Distance cardio. Think jogging, running, or biking at a steady pace for a long distance and/or time.
What is HIIT? - it stands for High Intensity Interval Training. Think jogging the curves and sprinting the straightaways on a track, running up hills and then walking down, 30 seconds of hard biking followed by 30 seconds of easy biking - alternating intensities.
Slow distance cardio has been regarded less well than HIIT recently in combat sports. Joel Jamieson is a proponent of using both - LSD to establish a base of aerobic capacity, and HIIT to improve your ability to function at higher intensities. This article makes a good case for LSD cardio for MMA fighters, and it also gives a great description of the ways your body adapts to each of them.