This is the third article talking about an Entity / Component proof of concept I've been developing. You should probably read the one talking about Components and the one talking about entities before.

In an Entity / Component architecture, there's data on one side (entities and components) and code that actually does stuff on the other side. One part of that would be the Systems.

Systems are mainly used to apply changes on a full set of entities like collecting all the Sprites components and drawing them on the screen. Typically, systems will be executed by order of priority.