I don't remember where i read it but i know i read a really neat article about ESC but this is what towerhobbies online rc dictanary
Electronic Speed Control (ESC) - Electronic speed controls replace the mechanical speed control and servo providing enhanced power efficiency and precision in an electric R/C car or boat. In addition, they are lighter which improves the performance of some electric models.
an esc is an electronic device that controls throttle instead of a servo. it does what a msc (mechanical speed control) does with a servo electronically