# Amazon Mechanical Turk core concepts
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The following are the core concepts of Amazon Mechanical Turk (Mechanical Turk) that you need to understand to use it effectively. 

## Requesters and workers
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 A *requester* is a company, organization, or person that posts tasks (HITs) to Mechanical Turk for workers to perform. A *worker* is a person who performs the tasks specified by a tequester in a HIT. 

## Marketplace
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 The [Mechanical Turk marketplace](https://worker.mturk.com) is where workers can go to find and accept tasks. In addition to the *production* marketplace, there is a second [*sandbox* marketplace](https://workersandbox.mturk.com) where requesters can post development tasks without money changing hands. 

More information can be found in Amazon Mechanical Turk marketplace. 

## Task or HIT
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The base unit of work in Mechanical Turk is called a *Human Intelligence Task*, which is typically designated as a *HIT* or *task*. A HIT represents a single, self-contained task, such as *Identify the color of the car in the photo*, that a requester submits to Mechanical Turk for workers to complete. 

Mechanical Turk is built around the concept of *microtasks,* which are small, atomic tasks that workers can complete in their web browser. When you submit work to Mechanical Turk, you typically start by breaking it into smaller tasks on which workers can work independently. In this way, a project involving categorizing 10,000 images becomes 10,000 individual microtasks that workers can complete. Hundreds of workers can work on portions of your project at the same time, which increases how quickly the work can be completed. In addition, you can specify that each task be completed by multiple workers to allow you to check for quality or identify biases in subjective questions. 

## Assignment
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When creating a HIT, you can specify how many workers can accept and complete each task. Doing so allows you to collect multiple responses for each item and then compare them. This additional information can be valuable in managing quality, as well as in collecting multiple data points when responses are subjective. 

When a worker accepts a HIT, Mechanical Turk creates an *assignment*, which belongs exclusively to the worker. The worker can submit results up until the expiration of the HIT. When retrieving results for a HIT, requesters retrieve all of the submitted assignments. 

## Reward and bonus
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A *reward* is the money you, as a requester, pay workers for satisfactory work they do on your HITs. A *bonus* is the amount you award workers for high-quality performance. Rewards are transmitted to workers when assignment submissions are approved, either by approving the assignment or when the auto-approval threshold is reached. Bonuses can be sent to workers who have recently completed an assignment for you. 

## Qualifications
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You can use *qualifications* to specify attributes of the workers eligible to work on your HITs. Qualifications can be either system-generated, such as qualifications based on location, or managed by you, based on past performance on your tasks. 

To learn more, see [Selecting eligible workers](SelectingEligibleWorkers.md).