# Concepts

> For the complete documentation index, see [llms.txt](https://docs.lertha.com/llms.txt).
> Any documentation page is available as raw Markdown by appending `.md` to its URL.

> The model behind the orchestrator, in the order it is easiest to learn.

Read these in order. Each page is short and ends with the one thing to remember.

1. [Flows](/concepts/flows): the durable program.
2. [Tasks](/concepts/tasks): the unit of work.
3. [Runs](/concepts/runs): ids, states, and the policies that make an id an idempotency key.
4. [Signals](/concepts/signals): how the outside world reaches a running flow.
5. [Timeouts and retries](/concepts/timeouts-and-retries): the two budgets.
6. [Determinism](/concepts/determinism): the one rule a flow body must obey, and why.
7. [Queues](/concepts/queues): how work finds a worker.

## The promise

**Durable execution**: your code keeps running through crashes, restarts and deploys, because
the orchestrator records every step it took and, after a restart, replays those steps to land
exactly where it was. Retries, timers, signals and queues are the machinery behind that one
promise.