In a collaborative effort with Scott over at Straight to the Bar, we will be writing all about rest for the month of October. You can expect a new article on the matter every week.
In general, there are only two types of rest that you can use during a workout. This is true in aerobic workouts such as running, swimming, or cycling as well as in anaerobic workouts such as weight lifting and yoga. Active rest tends to include any activity that keeps your heart rate elevated and/or your body in motion between sets or intervals. Passive rest is usually an activity that allows your heart rate to drop as quickly as possible.
When you are running, active rest can include activities such as jumping jacks, brisk walking, or running recovery between reps. For example, if you were doing half mile repeats on a track, you would warm up, run two laps fast as part of your repeat, and then jog a lap for recovery before starting your next repeat. Alternatively, you could do wind sprints, jumping jacks, pushups, or cartwheels while you wait for the next repeat to start.
Passive rest during the same workout would be some mild stretching, or leaning on a fence to catch your breath, or resting on your knees until it is time to start again.
If you were weight lifting, then you could use supersets and circuits as a form of active rest. By doing exercises back to back that do not use the same muscle groups, you can rest your muscles while continuing on your workout. This has the advantage of giving you an aerobic workout for an anaerobic activity, as well as cutting down on the total time needed to complete your workout. A circuit with four exercises back to back and one minute of rest between circuits for 3 sets will only have 2 minutes of passive rest, rather than 11 minutes if you were to do each exercise and then rest.