What is the difference between a gerund and a gerundive?

A gerund is a verbal noun which has an active force.

For example: Portare facile est = Carrying is easy, to carry is easy.

Gerund have no plurals, and decline in the other cases (where portare is nominative) by adding nd to the verb before and the neuter singular 2nd declension endings.

For example: Amphoram portando, nos vires crescimus = By carrying the jar, we are increasing our strength.

A gerundive on the other hand is a verbal adjective which always has a passive force and usually agrees with another noun. Unlike the gerund it can be plural and declines just like a 1st or second declension adjective.

For example: Amphoras portandas habeo = I have amphorae to carry.

Gerunds = verbal nouns, gerundives = verbal adjectives.