Annual Contract means , you need to purchase pay on monthly basis for one year , if you wish to cancel before completing one year , you will be charged Cancellation fees.
Monthly Subscription is for only one month.
Please refer below link
For example :
Creative Cloud full subscription - Annual plan = $49.99
Creative Cloud full subscription- monthly plan - $74.99
If you purchase as Annual Commitment , the price is reduced.
Monthly Plan: Pay month by month, cancel any time. No cancellation fee.
Monthly fees are higher than annual plan.
Annual Plan, Paid Monthly: 12 month contract (must pay for 12 months), paid monthly. Cancellation fee applies if you cancel before 12 months expires. Monthly fees are less than Monthly Plan.
Cancellation fee = 50% of all remaining monthly payments.