(This is a support stub for the Worlds and Planets submission.)
Easier to define is a planet. Quoting Wikipedia, as defined by the International Astronomical Union (IAU), it is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighbouring region of planetesimals.
Science aside, we approximately know what a planet is. What I call a world in this context, is (usually) a planet, inhabited by creatures of some intelligence, that have developed some degree of civilization. So our theoretical space-travelers would land on a planet, but visit a world, if you take my meaning (as a civilization usually trumps any interesting local features).
Now that I have some definitions, does this mean these are the only places fitting Worlds and Planets? Heck no!
First the obvious - there can be planets that are not worlds, as there are many more reasons to visit a place than pretty female aliens. Sadly the reasons are mostly scientifical. :)
Also, there can be worlds that are not planets… and here you can let your imagination run wild. Off the top of my hat, think Ringworld, or those extremely cold stars that could actually house organic lifeforms.
But note also the potential loopholes in the definitions - large moons of other planets may qualify as planet-like enough (consider Triton). Also, what if a ‘planet’ is not orbiting a star at all, even indirectly, but instead roaming the vastness of space on its own?
Well, you can probably get away with many a deviation here. But of course, the more exotic a post is, the more it deserves its own submissions.
See you planetside!