Plan
A plan is a one-time paid up-front fee and it has a specified duration of time.
*Plans will automatically cancel unless you pay the fee before the original duration of time expires.
Subscription
A subscription is a reoccurring price that would be billed either monthly or weekly to access a product/service for a certain period of time.
*Subscriptions are auto-renewable and will remain active unless you choose to cancel them via the iTunes store.