They will be charged for the monthly or annual subscription, depending on the option they choose, after the free 30-day trial is complete, unless they cancel before the next billing date.
• After the 30 day free trial, payment will be charged to your iTunes Account corresponding to the plan chosen.
• Any unused portion of a free trial period, if offered, will be forfeited when the user purchases a subscription to that publication, where applicable.
You can cancel any time during your 30-day free trial via the iTunes store settings and then you will not be charged.
Further, plans automatically renew unless you turn off auto-renew in your iTunes account 24 hours before the end of your current subscription period.