A lightweight object that has Components attached to it to describe it.
Using a Commands object received by a System, you can spawn a new entity by calling .spawn_bundle on it,
which will return an EntityBuilder to allow you to attach components to it.
Like a tag that goes on entities. They often do not need state of their own. It is common for a component to just be an empty struct. A System will usually operate on an entity that has a certain component attached to it.
Performs actions on Entities and Components. Systems are defined as functions, with the function arguments acting as a query. Bevy will call the system with any arguments that match the signature.
Data that is globally-set and unique to a certain Rust type.
For example, you could initialize a Materials struct that contains colors for rendering different things in the world.
A System then just needs to query for Res<Materials> in order to fetch that data.