O.K. I'll try...
Duty time...from the time you show up to work until some agreed upon time after your last flight.
Block...the actual flight time.
4:1 or 2:1...For every 4 (or 2)hours of duty time, you get paid one hour of flight time. Obviously 2:1 is a whole lot better than 4:1.
Per diem...An agreed upon hourly rate given to you while you're on duty to cover associated expenses, like food. At most airlines its about $1.50/hr, give or take a few cents.
These are a few of the reasons that when people talk about pay in the airline industry, hourly rates don't tell the whole story. They are oversimplified, but I hope it helps.
Thanks SSDD! Another queston, is per diem paid on every trip for every hour on duty, or block, and while you're sleeping on a 4 day trip?