Yep. The motherboard has a tiny little battery that keeps the CMOS and other important functions like system time going. Usually when it's plugged in but turned off, the console still draws a bit of power to keep it going, but when you have it unplugged it uses the battery. Being a battery, it only keeps charge for so long, so if you keep your console unplugged for too long, the system time gets reset.
It's much like a computer, but more simple.
I'm not.. nitpicking here but it's not really that much more simpler than a motherboard... I mean, you can run linux on it... And it looks physically similar to a motherboard.