Is starbound a single world with different characters?
What I mean is the following. If I have two characters, Derp and Herp, and I build a house with Derp on a given planet, will Herp be able to see the house also if they travel to that planet's coordinates?
Yes. The game has one universe and all your characters, and their buildings, exist in the same one. Anything you build will sync on the server or PC you build it on.