You got your two types of servo, see: your “regular” servo turns to the angle you ask for (by using a PWM signal). Your continuous servo turns at the speed you tell it to. It’s like this:
|regular servo||continuous servo|
|angle 0||full reverse|
|angle 90||don’t move|
|angle 180||full forward|
I’ve used two continuous servos to build a line following robot with differential steering.