There are four main types of running workouts: Fartleks/Speed work, Tempo, Pace, and Easy.
Fartleks / Speedwork
Speed work can be done as a solo workout or as a break in the middle of a longer easy run. Speed work days are shorter, but do not be fooled. These are still quality workouts!
An example of speedwork is doing workouts on a track. You may do 4 x 400 meters at your 5K pace for instance, with 800 meter jogs in between. Most training programs (running or triathlon) incorporate a speedwork day once every week or two weeks. These help to develop the fast twitch muscles so you have bursts of speed which can help you at the end of your race.
In part of a longer run, you may want to speed up from a certain point to the next tree then resume your pace for another mile or so, then speed up again until the red mailbox, then resume pace.
Any interval training where add bursts of speed to your workout is speed work! It can be as simple or technical as you make it.
Tempo
This is the thinking man's workout. The tempo run can be as hard or easy as you want to make it, and it has nothing to do with how long (in time) you run or how far. The goal is to start out at a moderate pace and work your way SLOWLY up to at or near 10K pace about 2/3rds of the way into your workout, then hold for two or three minutes and slowly returning over the last bit to a moderate pace.
We call this is thinking man's run because you are working towards a pace, maintaining, then backing down.
Pace
A pace workout is when you run a distance at your goal race pace. So if you wished to run a half marathon at 8:00 per mile, you would do you pace workout at 8:00 per mile.
Easy
Just as it sounds - run easy! If you're training with a friend, the two of you should be able to hold a conversation. If you can't do that, you're running too fast. (For those wearing heart rate monitors, your target zone should be between 65 and 75 percent of your maximum pulse rate.)
Your distance runs are to be run easy.