It is very painful for the cat, even in the recovery phase, Like someone said, it would be like removing a human fingernail but down as far as the first knuckle!
It also includes moving part or all of a smal bone, the terminal bone.
I'm not going to post any pics because they are pretty nasty, but thankfuly illegal in most european countries

It is estimated around 25% of cats in America are declawed

If anyone wants to read more about the subject here's a link:
DECLAWING: What You Need to Know

The sad thing is it is purely for convinience for owners,
My advice: Buy a scratch post!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!