In this video, PGA Professional Todd Kolb discusses which clubs in the bag are considered woods.  Basically, woods are characterized as the longest clubs in the set, they have a larger club head and have the least amount of loft.  The lower the number on the club, the longer the shaft and less lofted the club head.   Lessening the loft of the club creates a higher trajectory and less spin, which will allow the ball to carry further in flight and roll out longer after landing.

As an example, the driver would be considered the 1-wood, so it would be the longest club and have the least amount of loft on the club head.  These two characteristics allow for your driver to launch the ball at its highest trajectory with the least amount of spin, which gives you the most optimum distance in neutral conditions.  So going off of that logic, if you have a 3-wood and a 5-wood in your bag, the 3-wood will be smaller in size and shorter in length than the 1-wood with slightly more loft on the club, and the 5-wood will be shorter, smaller and more lofted than the 3-wood.