Kettlebells have an offset center of gravity due to the handle being on the top of the bell. Dumbells are balanced.
basically, anything you can do with a dumbell, you can do with a kettlebell. However, there are some things you can do with kettlebells that you can't do with a dumbell or a barbell.\
Kettlebells are great for doing ballistic moves (swings, snatches, etc.) These can build explosive strenght and cardio simultaneously...and do so with minimal risk to knees and back. You really cant swing a barbell. and Dumbell swings (especially at heavier weights) are pretty risky too.
Kettlebells are great for replicating the olympic lifts (clean, press, jerk snatch). The technique is easier to learn than with barbells (in my experience).
Kettlebell lifts tend to by compund lifts that hit much of the body. After jsut doing presses and swings...I feel it not jsut in my shoulder, but in my hamstrings, glutes, abs and lats. My workouts are also pretty brief (20-40 minutes).
Kettlebells are also portable. I was able to get by with just a couple of bells for the first year and a half of my training. I've very recently expalnded my collection as I'm working on some different techniques...but you can get by with jsut one or two bells and a basic program like Enter the Kettlebell for a long time.
depending on the program you follow, you can use bells to shred fat (geoff Neupert's KB Burn), build size (KB muscle or Return of the Kettlebell) or simply build conditioning and strength (Enter the Kettlebell).
I follow the dragondoor KB methods as taught by Pavel Tsatsouline. His methods also focus on proper use of tensions and relaxation to increase strength and power and protect the body (tensing abs, glutes and lats during a press for example). Karateka will find the tension methods very familiar.
If you go to
www.dragondoor.com and look at the kettlebell faq and some of the products and articles, i think you'll get a better idea of what kettlebells can do.
For me, I've been using them as the core part of my condiitoning for a couple of year now. I'm in the best shape of my life. Lost a lot of weight and bodyfat. My knee (wich if you recall had to have ACL reconstruction awhile back) no longer bothers me at all...and my old sholder/neck issues are pretty much a thing of the past.
I still do some barbell/dumbell and bodyweight work, but KB's are the core of what I do.
Peace,
Erik