A decade is 10 years and a century is 100 years. That’s the basic.
Both words originate from Latin. Decuria meant, division of 10 men and was used in the Roman military. Centuria meant a unit made up of 100 parts. That said the root of the word century comes from Saeculum, meaning 100 years. It is also used to define a cohort of individuals born at a particular time.
So in effect, we are talking about generational theory (Millennial, Gen Z, etc.). Think of a century as defining a long human life spanning approximately 4 generations, the passing of an era.