> ## Documentation Index
> Fetch the complete documentation index at: https://docs.vapi.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> The core building-block of voice agents on Vapi.

**Assistant** is a fancy word for an AI configuration that can be used across phone calls and Vapi clients. Your voice assistant can augment your customer support and experience for call centers, business websites, mobile apps, and much more.

There are three core components: **Transcriber**, **Model**, and **Voice**. These can be configured, mixed, and matched for your use case. <br /> <Info> There are also various other configurable properties you can find [here](/api-reference/assistants/create-assistant) </Info> Below, check out some ways you can layer in powerful customizations and features to meet any use case.

## Advanced Concepts

<CardGroup cols={2}>
  <Card title="Provider Keys" icon="key" href="customization/provider-keys">
    Add your API keys for other providers
  </Card>

  <Card title="Custom LLM URL" icon="brain" href="customization/custom-llm/fine-tuned-openai-models">
    Plug in your own LLM
  </Card>

  <Card title="Call Functions" icon="square-phone" href="/assistants/function-calling">
    Forward and hang up with function calls
  </Card>

  <Card title="Persistent / Temporary Assistants" href="/assistants/persistent-assistants" icon="database">
    Which setup is best for you?
  </Card>
</CardGroup>