New To Android - What is a launcher?

[Reformed IOS User]

I am new to android.
I have a 3GS and will be moving to Galaxy S4
I keep seeing "launchers" mentioned in the forum.
What exactly are they?

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Never mind found my answer

http://www.androidcentral.com/android-z-what-launcher

 

[meyerweb#CB]

Welcome to the light side.

A launcher is essentially a new home screen that replaces (in the case of Samsung phones) Touchwiz. Examples include Go Launcher, Nova, Apex, and ADW. I use Nova.

These launchers generally offer more flexibility and customization options than stock launchers, like more rows or columns of icons, more icons in the dock, etc.

 

[AMDZen]

Good information here. I also am looking to switch from iOS to android. Can anyone offer a website or something to see the visual changes these launchers offer? Can you always go back to Sense if i updated an HTC ONE from Sense to Nova?