Light turbulence = Your drink remains on the seat-back table without you having to hold onto it.
Meduim turbulence = You take your drink off the table and hold onto it.
Strong turbulence = Your drink ends up on the cabin ceiling.
The Captain deals with it by switching on the 'Fasten seat Belt' signs! This is backed up by an announcement from cabin staff. Some aircraft have CAT warning radar, so the Captain might alter course to avoid it. But some aircraft don't. If it becomes very bad, he might slow the aircraft, just as you would slow your car on a bumpy road.
Modern aircraft tend to be very strongly built, and can deal with pretty severe shocks, but this hasn't stopped them from being broken up in mid-air by CAT in the past.