Simple but effective - this breakout board has a CR1220 coin cell battery holder soldered on, an on/off switch and 0.1" pitch breakout pins for easy connecting. Great for powering very low-current projects, or adding a battery backup circuit to a real-time-clock or similar. There's two ground pins, ..
A real-time clock (RTC) is a computer clock (most often in the form of an integrated circuit) that keeps track of the current time. The term is used to avoid confusion with ordinary hardware clocks which are only signals that govern digital electronics, and do not count time in human units.