You really don't give much information of what you are looking for. For instance, what is your price, are you wanting a more luxurious atmosphere or more partying, will it be adults on the cruise or will there be kids, etc. Destination is also important.
But just to give you a basic idea I would say
For a smaller budget:
Carnival, there will be a lot of kids and younger adults (college aged) The boat has a Las Vegas feel to it, not quite as luxurious as some other lines, more for partying, the "fun ships" Not always the same standards as some other lines.
or
Royal Caribbean, there will still be kids onboard, but not nearly as many as Carnival. There are more adults onboard, and the average age tends to be a little older. A more classy, luxurious feel than Carnival, less partying.
Moderate budget:
Disney, Disney has some of the best service you can get onboard a cruise, it really is great. At the price they offer, you can't find the same service anywhere. The shows are great for anyone, especially Disney fans. There are a lot of kids onboard, but you mostly see them during character meet and greets, other times they are usually in the kids clubs. Very clean ships, good entertainment, great service and pretty good food. Mostly families onboard, but you will see a lot of couples onboard as well, without kids.
or
Princess, here were are starting to get into the lines that have a higher number of adults onboard. But, you will also find that the average age onboard is starting to rise. A very luxurious feel, classy, great food, clean, etc.
A larger budget:
Celebrity or Crystal, both are very high class. On these boats you are getting top of the line service, but it does come at a price. You will pretty much see only adults onboard. Some of the best food, service, amenities, etc. that you will find on a cruise.
or
A small yacht or something similar to Windjammer. These are smaller ships that only hold a few hundred guests. People onboard are looking to have a great time, let go and enjoy themselves. So, this is also a great ship if you are looking to party, but aren't really looking for something like Carnival.
Finally, I think that this link: http://cruisecritic.com/ will be helpful for you during your planning.