You may have found this out through the powers of google, but I've stolen this direct from another website as they sum it all up pretty well! (many thanks Runners World).
Quote:
Repetitions/intervals
Periods of hard running at 5K pace or faster, between 200m and 1200m in length, or 30 seconds and five minutes. Recovery periods can be short (30-90 seconds), or of an equal time or distance to the reps. Running at harder than race pace for short periods not only improves speed, but also allows you to work on your running form. When you're pushing hard, it's important to concentrate on things like arm and hand motion, posture and stride length. If you can keep these together during a hard session of reps, it will be easier to do so during a race. Don't attempt reps until you've tried other types of speedwork for a couple of months.
Tempo intervals
These are longer than ordinary intervals in that they take between 90 seconds and 10 minutes (or between 400m and two miles) and are run a little slower than your 5K pace. These work a bit like threshold runs - they raise the point at which lactic acid builds up in the muscles.
Fartlek
Fartlek is Swedish for 'speed play' and is the fun side of speedwork. Best done on grass or trails, you simply mix surges of hard running with periods of easy running. Run fast bursts between phone boxes, lampposts or trees when you feel like it, and as hard you like. Great for newcomers to speedwork.
Hills
Simple: find a hill that takes between 30 seconds and five minutes to climb at 85-90 per cent effort, and run up it. Jog back down to recover. A great alternative to track intervals.
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They're always something to consider, but realistically I would start to introduce them once your running at least 3 times a week and distance of 4-5 miles are the norm or can be done with ease... if your not at this point yet I think your training would benefit more from simply increasing your weekly mileage.
