The difference between annual and monthly billing in Pipedrive is the frequency at which the provided billing information will be charged with a Pipedrive invoice. You can look at our pricing page for more details on our plan options.
When you are on annual billing, subscriptions are charged once for the balance of one full year, and will not be charged again (assuming not alterations to the number of users in the Pipedrive account) until the year is over.
Being on the annual billing cycle will give you a discount compared to if you had opted for monthly billing.
When you are on monthly billing, subscriptions are charged on the same date each month every month, for as long as that subscription is active.