You've probably heard before that the human body is amazingly adaptable right?. Well that is true. In fact it is so adaptable that often times you become used to a certain type of workout (cardio, heavy lifting, endurance lifting, etc.). This is what we call a plateau.
Now in standard training methodology, the principle of overload should prevent the dreaded plateau. The overload principle simply states the the body most continually be challenged to promote further adaptation. For the common person this is generally interpreted as run farther or lift heavier. Often though, this does not provide enough stimulus to warrant change. So what is a person to do?
Change your workout style. Been lifting for hypertrophy (2-4 sets, 8-12 reps)? Try switch it up and lifting for power, typically 1-3 sets for 1-5 reps with heavy weight moving fast. Maybe you have been running long distance for years? Try adding some High Intensity Interval Training (HIIT) 1 or 2 times a week to break up the monotony. The key here is to force your body to re-adapt.
So am I saying here that you should do a completely different workout style every time you workout? Yes and no. If you do that, your body will become adapted to general fitness and be ready for anything. The downside of this is you won't be truly great at anything. This is fine for the average joe, but if say your training for a marathon, or looking for lots of hypertrophy, it would be better to give your body 4-8 weeks to adapt to the new workout, then switch back or move on to another style.
In doing this, you will "keep your body guessing," or forcing adaptation, which will yield greater results over time.