Strapped 

Strapped monitors provide an automatic, continuous heart rate reading from the chest strap to the wrist monitor. Because of this signal between the two pieces, the reading is more accurate than what is received from a strapless monitor. Strapped monitors can also be synched to certain software for in-depth tracking and analysis.  

Strapless

Strapless monitors are more comfortable and easier to use as they eliminate the hassle of wearing and properly positioning a chest strap. Users simply place two fingers on the sensor buttons on the face of the watch to receive a current heart rate reading. Some models have one-touch, single-button sensing. Strapless monitors are also more sanitary since there's no need to share straps that must be moistened with water or gel.