BLOCK time is actual "flight" time, usually defined as releasing the parking brake leaving the gate until they open the main door or the bag door at your destination (again, company dependent). CREDIT time is what you actually get paid. So if you get delayed, reassigned or cancelled, at the better companies you will still get paid for that segment, even though you didn't fly it (this policy is called "pay protection"). Credit time also includes deadhead (DHD) time and bonus pay. So lets say you get a couple of ground delays where you go sit in the airplane, a couple of deadheads in your month and work on a day off and you quickly can have 15-25 hours of "soft time" (the difference between what you fly and what you are paid) on your pay check. As you've heard many times, these policies vary quite a bit from company to company.