The Chief Operating Officer - is responsible for the day-to-day management and operation of the business. Doesn't carry as high a level of responsibility as the CEO - as it doesn't specifically cover direct accountability to shareholders, investors, etc.
Stands for Chief Operating Officer - responsible for the performance and management of the core business excluding the external investor relationships. Invariably gets fired for poor performance - and can be the 'fall guy' who gets the bullet to protect the CEO.