Arduino Uno Rev3 Pin usage (need help)

I am very new to robotics. However, I managed to get a 4WD platform working with the help of Arduino Uno Rev3 and Arduino Motor shield kit. Can someone please answer couple of basic questions for me?

  1. I read that digital pin 4, 7, 8 and 12 in Arduino Uno Rev 3 are used to drive the DC/Stepper motors via the latch. What is a latch?
  2. I believe that currently my motor shield uses pins 3, 5, 6, and 11 to control the (4) DC motors. What are the rest of the digital pins typically used for?
  3. Which are the pins that can be used by the rotary encoders?

Thanks in advance.

What are the rest of the digital pins typically used for?

What is a latch?

A latch holds (remembers) its state, in the same way a door latch holds the door open.

I believe that currently my motor shield uses pins 3, 5, 6, and 11 to control the (4) DC motors

They are some of the PWM outputs.

Thank you.