Per incident deductible: You pay the deductible every time you get a new medical ailment (be it sickness of accident related) before the insurance company pays anything.
Per visit deductible: You pay the deductible every time you you visit a health care provider (doctor, hospital, laboratory etc..) before the insurance company pays anything.
Per policy period deductible: You pay the deductible only once during the entire policy period, irrespective of how many times you get sick or injured during the policy period.
Annual deductible: You pay the deductible only once in a year irrespective of how many times you get sick or injured during the entire year.