Excalibur has a built in Entity Component System (ECS for short), which is a popular software technique in the video game industry for managing behavior in a game in a composable and reusable way. Many ECS implementations also focus on memory cache optimization techniques, that is not a goal of the current Excalibur implemenation.
The wikipedia article does a pretty good job of explaining the design pattern, but here is the gist:
Many of the features built in Excalibur are built using this, and users of Excalibur can make their own systems and/or replace existing excalibur systems.
In addition to the familiar Entity, Component, and System, Excalibur also implements an ECS World where all of these pieces live and can interact together. Each Scene in Excalibur contains an ECS world, they are not shared between scenes.
Inside an ECS World there is an EntityManager, QueryManager, and SystemManager that handle all the details.
Excalibur Actor's are in fact Entity's in this ECS implementation. Actors come "pre-installed" with a set of components and features.