In a sentence where the descriptive word follows a form of be (is, was, are, etc.), is the description an adjective or an adverb?

I.E. Arnold is stronger than Sylvester. (stronger) or simply: Arnold is strong. (strong)
The Corvette was faster than the Porsche. (faster)

It's been a long time since I had fifth grade English! Thank you.
I think these are called comparatives.
See; http://www.english.uiuc.edu/cws/wworkshop/grammar/comparatives_and_superlatives.htm
As mike said, it is a comparative. Adjectives are the base form for making comparatives.