DOMS
DOMS—or delayed onset muscle soreness—is as much of a downer as it phonetically sounds. It's the pain you might feel 24 to 48 hours after a run. Do some active recovery, like walking or dynamic stretching to help prevent DOMS, and try eight ounces of tart cherry juice, which has been shown to help reduce post-exercise muscle soreness and inflammation.
More: How to Use Active Recovery to Improve Your Running
DNF
This one's used in all forms of racing, and it's something runners really try to avoid: Did Not Finish. A participant gets a DNF when not finishing a race—it could be because of injury or fatigue.
More: When Should You Drop Out of a Marathon?
Fartlek
This Swedish word sounds silly, but it actually means speed play. As a training modality (think unstructured interval training) it's kind of fun. Try it: Sprint—as fast as you can—for alternating distances and times.
More: Fartlek Workouts to Elevate Your Fitness
Tempo
This type of run is also known as a lactate-threshold run, and it's when your pace is just hard enough to be challenging—yet not punishing. Tempos are usually two to four miles in length and can be run as intervals, with jogging or even walking in between.
More: What Pace Should Runners Run Lactate-Threshold Workouts?