Now, with any subject, there are bound to be some debates. And one of the biggest I've found in the ship world is the following: Is this ship an ocean liner, or a cruise ship? And there's plenty of back and forth on this, like some people say that Queen Mary 2 is an ocean liner, and some say she's a cruise ship. But there are several ways to tell which is which, and I'll list them here (though there are plenty of exceptions.) 

1. Structural difference. Ocean liners are built for speed, so they will generally be built a bit more streamlined, and with thicker, stronger, reinforced hulls, for braving the high seas. They also sit lower in the water. 

2. Purpose. Although these ships may seem similar, they serve very different purposes, and are catered to different markets. An ocean liner's main job, is to transport people from place to place as fast as possible, while cruise ships are the destination; the ship is the main experience.

3. Size. Most of the time, cruise ships will be larger overall, than ocean liners, as they are designed for capacity, not speed.

4. Is it the Queen Mary 2? Because any ship still sailing today that isn't the QM2, is not an ocean liner. Even if it looks like one. Take the Disney cruise line. They have ships that are designed to look like 1930s ocean liners, but are still very much cruise ships.