what's the difference between shudder and shiver?
when they used to describe shake

Question

Answer

Shiver- to shake slightly because of the cold.
Shudder- to shake slight due to any other feeling or emotion (anger,fear etc.)

Answer

Shudder: to tremble convulsively, typically as a result of fear or repugnance. 
Shiver: to shake slightly and uncontrollably as a result of being cold, frightened, or excited.
They can mean the same thing.