Multi Window is a new feature that allows you to use two apps in the same time, but not in the old multitasking way, but in the same time on the same screen. You can watch a YouTube video and text a message in the same time using Multi Window. However, when Samsung introduced this feature, it only enabled it for a limited number of apps. So therefore you cannot use multiwindow for any app out there. Luckily, developers have been working on this issue and came up with an app that allows to to use any app with the feature.MultiWinodw Manager will allow you to use any app in Multiwindow. However you will only be able to install this on a rooted ROM and on Jelly Bean 4.1.2. Also MultiWindow Manager only works for Galaxy S3, S4, Note, Note 2, Note 8.0 and Note 10.1. You cannot install it on other devices.You can free download MultiWindow Manager from Google Play. Install it and then follow the instruction in the video below if you want to learn how to use the app. Multi Window is a new feature that allows you to use two apps in the same time, but not in the old multitasking way, but in the same time on the same screen. You can watch a YouTube video and text a message in the same time using Multi Window. However, when Samsung introduced this feature, it only enabled it for a limited number of apps. So therefore you cannot use multiwindow for any app out there. Luckily, developers have been working on this issue and came up with an app that allows to to use any app with the feature.MultiWinodw Manager will allow you to use any app in Multiwindow. However you will only be able to install this on a rooted ROM and on Jelly Bean 4.1.2. Also MultiWindow Manager only works for Galaxy S3, S4, Note, Note 2, Note 8.0 and Note 10.1. You cannot install it on other devices.You can free download MultiWindow Manager from Google Play. Install it and then follow the instruction in the video below if you want to learn how to use the app. 
What's my model number?
There are several ways to locate your model number:
- Option 1
- On your device, go to Settings, then "About device" and scroll down to "Model number"
- Option 2
- Often times you can view the model number inside the device, by removing the battery
- Option 3
- Using Samsung's model/serial number location tool