Evapotranspiration accounts for the amount of water lost to the atmosphere as a result of evaporation (both from soil and plant leaves) and transpiration from the plant issues.
Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies.
Transpiration accounts for the movement of water within a plant and the subsequent loss of water as vapor through stomata in its leaves.
Evapotranspiration is an important part of the water cycle. An element (such as a tree) that contributes to evapotranspiration can be called an evapotranspirator. (source: en.wikipedia.org/wiki/Evapotranspiration)
EvapoTranspiration (ET) formula used by RainMachine:
based on a ASCE (American Society of Civil Engineers) 2005 Final Report study:
The ASCE Standardized Reference Evapotranspiration Equation 2005 (PDF)
The ASCE Standardized Reference Evapotranspiration Equation 2005 Appendix (PDF)