Ignition Fuel Tools provides both a command line tool and a C++ API which allow interacting with Ignition Fuel web servers such as https://app.ignitionrobotics.org and their hosted simulation resources.
Simulation resources
The Ignition Fuel web server hosts several resource types relevant to robotics simulation, such as models and worlds.
Models
A common asset stored in an Ignition Fuel server is a model. A model represents an object in simulation. A model has a name and a description of its visual and physical properties. Models can be as simple as a sphere or complicated as a highly articulated humanoid robot. See the full model specification here.
Worlds
Among the resouces hosted at Ignition Fuel there are "worlds", which describe a everything inside a simulation, from the robots to the scene properties, lights, physics properties, etc. You can see the full world description here.