I'm going to get right to the point, rather than go on about destinations, embankment, cabins, entertainment, etc. There is much written about this, and most of it is standard for the line and industry.
Pros: Carnival gets a bad rap for being a budget cruise line, but their service, food, and accommodations are fantastic for the price. While they might not match the premium cruise lines, they are far above Norwegian (at least in these areas).
Cons: Be careful, and don't expect Carnival to have your back. Their shops are usually more expensive than stateside, their booze will be cheaper at port, and their art auctions (Park West Gallery) have been caught selling frauds (especially Peter Max). Even worse, their shopping seminars are meant to completely deceive you, including crippling investment. I met a guy that put his savings into tanzanite gems at $200 a carat, even though you can buy them at Macy's for $100.
It's unfortunate that Carnival makes money off deceiving people, but if you keep that in mind, you'll have a great time for the money.