Undervolting means the phone will use less power to run so you will see battery improvement.
Overclock runs the cpu at higher clock speeds so that it can work faster but that decreases battery life. OC with undervolt should negate the battery loss.
Underclock is the opposite and it increases battery life and combine with UV makes battery life better, but of course the max clock speed is lower so it is slower.
The governor controls how fast the cpu's clockspeed is increased when it senses more load, and how fast it decreases clock speed when there's less load. It also controls when the second cpu is activated as well as when the cpu goes to sleep. Some governors ramp up the clock speed to max almost immediately when it senses any bit of load so you get max performance, but since the cpu is almost always on max speed it drains the battery faster. Other governors ramp up clock speed slowly so the cpu is rarely at max speed and this saves battery but performance isn't as great. And others are a combination of the two.
