A pilot is allowed to be on duty up to 16 hrs. Some contracts lower that number, but we'll use 16. You fly a trip, usually one leg out of domicile at night and back in the morning, all in less than 16 hrs of duty. You receive less than 8 hrs of rest in between legs, making it continuous duty. Usually the company provides a hotel in between, good for a 4-6 hour nap. You're off all day the next day, providing you legal rest to go do it again.
Example:
You report for work at 22:00, fly at 23:00 for 1.5 hrs, putting you at the hotel around 01:00. You have a 05:00 show the next day to fly home. Should release from duty around 08:00. You have been on continuous duty for ten hrs. Basically, you're working the graveyard shift.
Some companies have limitations on duty, minimum time on the ground, duty rigs, limits on number of legs. Obviously, in my example, you would only be flying 50 hrs or so per month so a duty rig is greatly desired to up the pay. At CMR, you get 2:1, duty to pay. So, if you did the above trip (10 hrs duty) 16 days a month, you would have 160 hrs of duty and be paid for 80 hrs. Clear as mud?