River cruise ships tend to be much smaller than ocean vessels. European river cruise ships, in particular, are relatively narrow and compact because they need to be able to pass through locks and under bridges. This means that you will share your journey with fewer passengers. It also means that there are fewer shipboard activity areas; if your idea of a great cruise vacation depends on the availability of multiple restaurants, spectacular shows and an all-night casino, a river cruise might not be your ideal getaway.
Some river cruise ships are so small that they don't even offer a self-service laundry or fitness center. Your meals will be well-prepared and beautifully served, but you will probably have only one or two dining venues onboard your river cruise ship.