Overtraining is a common misconception in health and fitness. I think a lot of people have fallen prey to the idea that they're doing to much so they actually stop doing enough and do too little.
It can get confusing, but I truly believe that it's far more beneficial to train more often than not. I train 6 days a week myself and take the 7th as an off day. The funny thing is that there are many people who train 3-4 days a week and think that if they do 1 more day, they would be overtraining and making "negative gains."
I really don't buy into that story, unless maybe you're working out 6 hours a day each of those 3 or 4 days... But you're probably not doing that.
We have to keep in mind that our bodies adapt to the conditions we put them in. If we keep these conditions easy, like sitting in a chair for hours and hours on end for example... Then we run the risk of looking like a person who sits in a chair for hours and hours on end.
If however, we decide to do something about that and become a person that works out for 2 hours a day, 6 days a week, then we will look like a person that works out for 2 hours a day, 6 days a week.
If you never do any cardio, you may look like a person who never does cardio and is a little on the bigger side: your body has adapted to the conditions you put it in. If you start running a lot, your body will realize that the excessive weight is inefficient and thus, it will burn it off (provided you're not taking in a surplus of calories).
Overtraining all comes down to how you feel: if you find that working out 5 or 6 days a week is just too much for your body and you're constantly sore and fatigued, then change your regime till you find what works for you.
How many days a week do you all train?