Pictured above is the Lodge straight kettle. They're discontinued and very hard to find.
They look like dutch ovens, but are more narrow and deeper than the dutch ovens.
They were designed for slowly simmering foods on low heat. The shape allows for better fuel efficiency due to less heat.
For outdoor cooking, they're meant to be hung over an open fire, as opposed to the camp oven, which sits on hot coals.
Pictured is the flat bottom kettle, which could be used on the stovetop as well as over a fire. The round bottom kettle could only be used over a fire.
They're sometimes called "bean pots", since beans are simmered slowly on low heat.
If you'd like to cook over an open fire, not to worry; Lodge still sells dutch ovens with wire handles for hanging. And anything you can cook in this kettle, you can cook in a dutch oven.
So... I'll stay happy with my dutch ovens.
I think the flat bottom straight kettle looks exactly like a cast iron maslin pan.
I have no idea what the difference is, except that the maslin pan has a side handle for tipping.