Inspired by the buggy nature that campfires appear to show, I decided to test their properties.
This testing was done using a clean copy of DF 0.47.05 with no DFHack or any other modification
A small world was generated for 125 years of history. 40 artifacts and plenty of necromancer towers.
I have split my results into three categories using a Q/A style format. The categories are:
Placement
Q: Can a campfire be placed on a tile occupied by a creature?
A: Yes. Although the campfire is a movement-blocking tile, the menu still gives you the option
Q: Can a campfire be placed on furniture?
A: Yes. Tested with doors, cabinets, slabs, statues, workshops, altars, display cabinets, and pedestals. Even if the furniture tile blocks movement, a campfire can be placed there
Q: Can a campfire be placed on constructed floors?
A: No. Campfires appear to need pure ground. Constructions of any kind block campfire placement.
Q: Can a campfire be placed on ice?
A: Yes, although it doesn't melt the ice like one would expect.
Q: Can a campfire be placed on leaves?
A: No. It appears that campfires need solid, non-modified ground to be placed on
Q: Can a campfire be placed on slopes?
A: Sadly, no
Movement
Q: Can you move into a campfire in any way?
A: Yes. By jumping or falling into it, you can land on it. It has no negative effects however.
Q: Can other creatures move into campfires?
A: No. You can trap other creatures in campfires too, but it causes some serious lag.
Q: Can you or other creatures dodge into campfires?
A: Yes. Like before, no ill effect
Interaction
Q: Can campfires destroy items when dropped into them?
A: Yes. Any item that gets dropped into one gets destroyed.
Q: What about decorated items?
A: I dropped a sock with a sewn image on one, and it disappeared. Likewise, I also dropped a heavily decorated steel mace on it and it too disappeared.
Q: What about furniture items?
A: I toppled a statue and dropped it onto the campfire. Nothing can survive the awesome power of the campfire.
Q: What about corpses?
A: I disposed of a local monk and threw him into the campfire. His remains disappeared instantly. More science will be required to see if he disappears in legends.
Q: What about items thrown from a height?
A: It works the same way as dropping while standing on the campfire.
Q: What about artifacts?
A: Here's where it gets interesting. From my testing, all artifacts that fall into the campfire are destroyed. Here's how I know.
I found a necromancer tower, and slaughtered it's undead defenders. It so happens that the tower contained the original secrets slab artifact created by the deities themselves. I learned the secrets of life and death from it, then tossed it into the campfire. When I went to Look at it, the slab was gone. I repeated this test with other unique copies of books, which all disappeared. Finally, I tried it out with a bone decorated mug from a nearby human village, and it too vanished. Looking at legends, the history of the artifacts end lines like ""Tombsbury the Phantom of Skulls was destroyed in Pillarplane."
In conclusion, campfires are impassable constructed floor tiles that ignore furniture placement and destroy any item thrown into them. It doesn't spread it's heat downwards into the floor, and shields all other creatures from harm with some kind of magic.