What are grip shifters?

Twist grip disc: Is a gear lever on the handlebars that is integrated into the handlebar grip, similar to the throttle on a motorcycle. Turning it changes the cable tension and therefore the gear engaged on the gearshift.

Twist grip switch (GripShift):
Is a gear lever on the handlebars that is integrated into the handlebar grip, similar to the throttle on a motorcycle. Turning it changes the cable tension and therefore the gear engaged on the gearshift.

Relevant articles:
Adjusting the rear derailleur , adjusting the bicycle gears