- fietsen i.e. literally "bicycling" would refer to riding an opafiets [1] bicycle, at moderate speed, without helmet or any sort of cycling attire;
- wielrennen, i.e. literally "wheel running" instead describes sort of cycling that you would see in Tour de France race - racing bike, helmet, lycra gear on, considerably faster.
Second is far less common, and doing so on busy city streets during rush hour is frowned upon, of course.