When you turn on (or unlock) your Android device you will be presented with a screen similar to below, this is the home screen (or launcher), what you would think of as a desktop if you are a Windows user
This can be fully customized by changing the background image, adding/removing icons etc
Please bear in mind that a lot of devices running Android are heavily customized by the manufacturer so this home screen may look different depending on your device

Notifications |
Notifications are shown when the system or an app needs to notify you that something needs your attention, such an email message, text message, Facebook alert or update information
Swiping down from the top of the screen will then show you more details on any current notifications, as well as provide access to the Quick Settings panel |
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Status Bar |
This area shows important information about your device such as time, wireless signal strength, battery level etc |
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Back |
This works in the same way as the back button in your web browser and takes you back to the previous page you were working on |
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Home |
The home button returns you back to the home screen as shown above |
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Recent Apps |
Clicking this shows you any recent apps you have used to allow you to quickly switch back to them or to close them completely |
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Apps |
This opens up a screen showing you all applications installed on your device |
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Open Apps |
Icons are shown here for any apps you currently have open, clicking the icon takes you to that app |
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Dock |
This area which is sometimes referred to as a favorites tray allows you to "pin" your most used apps so you can access them quickly, icons can also be dragged on top of each other to create groups or folders of apps |
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Search |
Google Search - type here to search the internet using Google |