---
title: Concepts
---

This area contains explanations and guidance on the concepts in Cratis.

| Topic | Description |
| ------- | ----------- |
| [Event](/chronicle/concepts/event/) | What constitutes an event |
| [Event Type](/chronicle/concepts/event-type/) | What is an event type |
| [Event Source](/chronicle/concepts/event-source/) | What is an event source |
| [Event Store](/chronicle/concepts/event-store/) | What is the event store |
| [Namespaces](/chronicle/concepts/namespaces/) | What are namespaces within an event store |
| [Event Sequence](/chronicle/concepts/event-sequence/) | What is an event sequence |
| [Event Metadata Tags](/chronicle/concepts/event-metadata-tags/) | Metadata tags attached to events |
| [Observer](/chronicle/concepts/observers/) | What is an observer |
| [Projection](/chronicle/concepts/projection/) | What is a projection |
| [Tagging](/chronicle/concepts/tagging/) | How to tag events for organization and filtering |
| [Tagging Reactors](/chronicle/concepts/tagging-reactors/) | How to use tags with reactors |
| [Constraints](/chronicle/concepts/constraints/) | What are constraints |