Fartlek: This is a Swedish word that means “speed play.” Fartleks are runs in which you alternate easy running with moderate to hard efforts. They are typically unstructured runs that are intended to switch things up, raise your heart rate, and aren’t tethered to any specific pace or time goal. These are fun runs to do in a group or a good way to keep things interesting on a solo run.
Tempo: This is kind of like a workout sandwich, and it’s similar to a fartlek but with more structure. A tempo run sandwiches harder running (the meat) between a warm up and cool down (the bread).
The “meat” of the tempo run is at, or slightly above, your anaerobic threshold—the effort level just above your comfort zone. The pace should feel challenging enough that you cannot have a comfortable conversation while running, but you can get a few words in without gasping for breath. You may have a specific pace to shoot for or simply go by feel and effort.
Workout: A session of vigorous effort or training that's typically used to describe a speed workout or a hill workout. This is where you might work in those intervals or fartleks. Something more than a regular training run.