Basic documentation for Entities, Components and Systems (#1578)

These are largely targeted at beginners, as `Entity`, `Component` and `System` are the most obvious terms to search when first getting introduced to Bevy.

crates/bevy_ecs/src/component/mod.rs

@@ -11,6 +11,19 @@ use std::{
 };
 use thiserror::Error;
 
+/// A component is data associated with an `Entity`. Each entity can have multiple different types
+/// of components, but only one of them per type.
+///
+/// Any type that is `Send + Sync + 'static` automatically implements `Component`.
+///
+/// Components are added with new entities using [Commands::spawn](crate::system::Commands::spawn),
+/// or to existing entities with [Commands::insert](crate::system::Commands::insert),
+/// or their [World](crate::world::World) equivalents.
+///
+/// Components can be accessed in systems by using a [Query](crate::system::Query)
+/// as one of the arguments.
+///
+/// Components can be grouped together into a [Bundle](crate::bundle::Bundle).
 pub trait Component: Send + Sync + 'static {}
 impl<T: Send + Sync + 'static> Component for T {}
 

crates/bevy_ecs/src/entity/mod.rs

@@ -13,7 +13,11 @@ use std::{
 
 /// Lightweight unique ID of an entity
 ///
-/// Obtained from `World::spawn`. Can be stored to refer to an entity in the future.
+/// Obtained from [World::spawn](crate::world::World::spawn), typically via [Commands::spawn](crate::system::Commands::spawn). Can be stored to refer to an entity in the future.
+///
+/// `Entity` can be a part of a query, e.g. `Query<(Entity, &MyComponent)>`.
+/// Components of a specific entity can be accessed using
+/// [Query::get](crate::system::Query::get) and related methods.
 #[derive(Clone, Copy, Hash, Eq, Ord, PartialEq, PartialOrd)]
 pub struct Entity {
     pub(crate) generation: u32,

crates/bevy_ecs/src/system/system.rs

@@ -17,6 +17,15 @@ impl SystemId {
 }
 
 /// An ECS system that can be added to a [Schedule](crate::schedule::Schedule)
+///
+/// Systems are functions with all arguments implementing [SystemParam].
+///
+/// Systems are added to an application using `AppBuilder::add_system(my_system.system())`
+/// or similar methods, and will generally run once per pass of the main loop.
+///
+/// Systems are executed in parallel, in opportunistic order; data access is managed automatically.
+/// It's possible to specify explicit execution order between specific systems,
+/// see [SystemDescriptor](crate::schedule::SystemDescriptor).
 pub trait System: Send + Sync + 'static {
     type In;
     type Out;