What is the difference bewteen a gerund and a gerundive?

Technically speaking, a gerund is a verbal noun and a gerundive is a verbal adjective. Gerunds and gerudnives both look very similar - the key feature in the words is a 'nd'. For example: amandi, vivendo, portandum.

What does this mean in practice?

To translate a gerund, the english word will usually end in 'ing' - e.g. walking, singing etc. Gerunds are used whenever you need to make a latin verb into a noun. For insance : ars amandi est difficilis = the art of loving is difficult.

Gerunds can also be used to express purpose - lego ad discendum = I read in order to learn

Translating a gerundive is a little more tricky as there is no direct english equivalent. However, the best way would be to translate it in the passive form 'to be --ed' e.g. to be carried, to be considered etc. When gerunds are found with the verb 'sum' there is a great sense of obligation (this is called the gerundive of obligation). For example: amandus est - he must be loved. Karthago delenda est - Carthage must be destroyed. Due to this sense of obligaiton, very often gerundives are present in sentences with the word 'causa' - so whenever you see a word that looks like a gerund or a gerundive with the word causa, you know it will be a gerundive. We translate 'causa' in this instance as 'for the sake of'. Example: puer vivit puellas basiandi causa - the boy lives for the sake of kissing girls

When the gerundive is used by an agent, it usually takes the form of ab + dative (this is a good way to spot whether a word is in fact a gerundive, as normally  passive verbs take an ablative agent!) For example: puer amandus est puellae - the boy must be loved by the girl.